Colossus vs Marines is closer to the hard counter idea, but Flame Tank vs Riflemen makes the Colossus vs Marine fight look fair.

The "Stealth vs Detection" thing isn't quite the same. When you have a mixed-army (ex: Light Tanks + Flame Tanks) go up against another mixed army (ex: Riflemen + Rocket Launchers), the "Hard Counter" remains in fact, a hard counter.

No amount of "support" will allow those Riflemen to damage the Flametank in any reasonable manner. (And Light Tanks may not be able to damage Riflemen, but Riflemen can't damage Light tanks either). The few rocket soldiers you have allow some degree of DPS, but its just a matchup destined to lose no matter the support you put into the mix. In all situations in all manners of various support and/or mixed army compositions, the Riflemen will be useless against Flame Tanks.

Riflemen don't do much damage (maybe 1HP??) to Flame Tanks. Riflemen are slower than Flame Tanks, while Flame Tanks one-shot Riflemen with a large area-of-effect damage. Flame Tanks also kill Riflemen by movement (if you move into a squad of Riflemen, they all get squished before the flames even come out). The Riflemen have to constantly micro away from the Tank's movement to remain alive.

Effectively: imagine if those Colossus in SC2 had armor that only took 1-dmg from Marines, had special code to "step" on Marines to kill them (aka: instant-kill any marines that are under the Colossus), and had a movement speed roughly 2.5x faster than Marines. Also imagine that Marines didn't have stimpacks. Colossus retain the area-of-effect lasers, except the lasers one-shot marines. And you're beginning to see how hopeless Flametank vs Riflemen look like in C&C.

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In contrast, 12 Zealots will probably beat 12 Dark Templar (or roughly go even there-abouts) as long as detection is nearby. Its a very different mechanic from the C&C-style "Hard Counter".

A "Hard Counter" remains a counter, even in the presence of supporting units.

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Starcraft 1 / Starcraft 2 just aren't as "counter-heavy" as other RTS games. In Age of Empires: Scouts lose vs Pikemen. Archers lose against Scouts. Castles lose to Trebuchets.

Age of Empires 2 isn't as "hard counter" as C&C, but its closer to the spirit of C&C's hard counters than anything in the Starcraft / Starcraft 2 world.

Case in point: A 4800 HP Castle shoots 4-arrows of 11-damage against 35-HP Crossbowmen who shoot only 1-arrow of 1-damage back in the same time period. (The 8-damage a Crossbow normally deals is turned into only 1-damage vs stone-structures like Castles or Walls) The Castle "Hard Counters" Archers. There's no composition where an archer would ever be useful at anti-Castle warfare in Age of Empires 2.

(Castles don't truly "counter" Archers however, because Castles can't move. Within the scope of mobile armies, various units get random bonuses against each other however, which plays somewhere in between SC2's soft-counter style vs C&C's hard-counter style. Nonetheless, strong players will bring 20 villagers to combat if only to build a castle during a fight, because of the ability for a "pop-up Castle" to beat so many different kinds of units. Especially in the Castle Age... the "Hard Counters" for Castles is locked away in Imperial Age (Trebuchets and/or Bombard Cannons))