There is a difference between the two, but the confusion exists because some high-frequency words can also be sight words. High-frequency words are simply those that appear a lot in common speech like number words or articles.
Many fit standard phonetic pronunciation, but some do not. That is where the overlap occurs.
Sight words must be memorized. Many sight words appear frequently in texts that students read, but not all sight words are high-frequency words. The word rhythm is a sight word, but doesn’t necessarily appear a lot unless you’re a musician.