Question: Are both Crispian Mills and Billy Gibbons considered singers?

Context articles: A Fantastic Fear of Everything
Perfectamundo
The Jeevas
Social Distortion Winter 2012 Tour
Moving Sidewalks
Crispian Mills
Billy Gibbons
Kula Shaker
ZZ Top
Got Me Under Pressure

A Fantastic Fear of Everything is a 2012 British horror comedy film starring Simon Pegg, written and directed by Crispian Mills with Chris Hopewell as co-director.
 It is based on the novella "Paranoia in the Launderette" by Bruce Robinson, writer and director of "Withnail and I".
 It has been described as a low-budget "semicomedy" about a children’s author-turned-crime-novelist who has become obsessed with murder and murdering.
 It was released on 8 June 2012 in the United Kingdom and Ireland, and received a limited U.S. theatrical release on 7 February 2014.
 The BBFC classified the film a 15 certificate in the UK, while the MPAA rated the film R in America.
Perfectamundo is the debut studio album by American rock musician Billy Gibbons and rock band The BFG's.
 The album was released on November 6, 2015, by Concord Records.
The Jeevas were an English rock band.
 Its members were Crispian Mills (vocals, guitar), Andy Nixon (drums), and Dan McKinna (bass).
 Mills was previously the vocalist of Kula Shaker.
 Nixon and McKinna were previous members of Straw.
 Mills rejoined Kula Shaker in late 2005, and The Jeevas disbanded.
The Winter Tour (also known as "House of Blues Residency" was a tour by Social Distortion.
 As its name applies, most of the shows were based out of the House of Blues in Orange County (Anaheim, San Diego, West Hollywood) with a couple of shows heading out to Las Vegas and Tempe, Arizona.
 Even though similar, this tour was not a part of the Hard Times and Nursery Rhymes tour, as most of the material played here was pulled from most of the band's catalog.
 This is also not the first time Social Distortion has done a tour like this, it has been done before numerous times since 2001.
 As with previous House of Blues tours, the band played rare songs and new songs and brought out guests this time, such as Billy Gibbons and Mike Ness' own son Julian Ness.
 They also brought out Eric "Jailhouse Blues" Von Herzen who played harp for "It Coulda Been Me" and "Drug Train" on Social Distortion (album) and they also played ZZ Top's La Grange (song) with Billy Gibbons.
 The band also premiered songs like "Far Side of Nowhere", "Writing On the Wall" and performed songs that have not been played in a while like "It Coulda Been Me", "99 to Life", "Footprints On My Ceiling", "Hour of Darkness", "Diamond In the Rough" and even played the White Light, White Heat, White Trash album in its entirety at some shows.
The Moving Sidewalks was an American psychedelic blues rock band, most notable for giving future ZZ Top guitarist, Billy Gibbons, his start in the music business.
 The band consisted of Gibbons on guitar, Don Summers on bass, Dan Mitchell on drums and Tom Moore on keyboards.
Crispian Mills (born 18 January 1973 as Crispian John David Boulting), also known as Krishna Kantha Das, is an English singer-songwriter, guitarist, and film director.
 He is the son of actress Hayley Mills and director Roy Boulting, the grandson of Sir John Mills and Mary Hayley Bell, nephew of Juliet Mills and director Jonathan Mills, and half-brother to Jason Lawson.
William Frederick Gibbons (born December 16, 1949) is an American musician, singer, songwriter, producer, and actor, best known as the guitarist and lead vocalist of the American rock band ZZ Top.
 He began his career in the Moving Sidewalks, who recorded "Flash" (1968) and opened four dates for the Jimi Hendrix Experience.
 Gibbons formed ZZ Top in late 1969 and released "ZZ Top's First Album" in 1971.
 The albums that followed, "Rio Grande Mud" (1972) and "Tres Hombres" (1973), along with extensive touring, solidified the group's reputation as a hard-rocking power trio.
Kula Shaker are an English psychedelic rock band.
 Led by frontman Crispian Mills, the band came to prominence during the Post-Britpop era of the late 1990s.
 The band enjoyed commercial success in the UK between 1996 and 1999, notching up a number of Top 10 hits on the UK Singles Chart, including "Tattva", "Hey Dude", "Govinda", "Hush", and "Sound of Drums".
 The band's debut album, "K", reached No. 1 on the UK Albums Chart.
ZZ Top is a rock band formed in 1969 in Houston, Texas.
 The band has, since 1970, consisted of bassist and lead vocalist Dusty Hill, guitarist and lead vocalist Billy Gibbons (the band's leader, main lyricist and musical arranger), and drummer Frank Beard.
 "As genuine roots musicians, they have few peers," according to former musician, critic and collector Michael "Cub" Koda.
 "Gibbons is one of America's finest blues guitarists working in the arena rock idiom [...] while Hill and Beard provide the ultimate rhythm section support."
"Got Me Under Pressure" is a song by ZZ Top from their 1983 album "Eliminator".
 The song was produced by band manager Bill Ham, and recorded and mixed by Terry Manning.
 David Blayney (ZZ Top's stage manager of 15 years), in his book "Sharp Dressed Men" described how the song was pre-produced: Billy Gibbons and Linden Hudson (Houston engineer and songwriter) wrote the whole song and created a recorded demo all in one afternoon without either Dusty Hill (ZZ Top bass player) or Frank Beard (ZZ Top drummer) even knowing about it.
 Linden created the bass on a synthesizer, created drums on a drum machine and helped Billy Gibbons write the lyrics; Billy performed the guitars and vocals.