Not true. Tails cannot go super in Knuckles in Sonic 2. (this can be found via setting your character to Tails, despite it being for "Knuckles")
Tails is not in S2K. The options menu isn't accessible. Even if he was, my point is that S&K and the lock-on modes are the only time in the classic games when non-Sonic characters can go super.
If Sonic Advance let you go Super Sonic in regular stages and not Knuckles, I would have been upset there as well, because Knuckles had a super form. It bothered me in Sonic Heroes when Knuckles was just Knuckles with a power shield. The thing is there. We know it exists. By not giving it to us it's kind of like taunting us. I could let Tails and Amy slide because Tails NEEDED the Hyper Emeralds and Amy has never gone super.
I think everyone seems to have a misunderstanding about what super is and
why SEGA changed how they use super after S3K. I think the why is SO much more important here than the specifics of the implementation in S3K.
In Sonic 2, they introduced Super Sonic as an easter egg reward for collecting all the chaos emeralds. You'll notice that you can't prevent it from activating in Sonic 2, and it's very much just a hidden secret. If you play as Tails, you get nothing, and you're none the wiser.
In Sonic 3, however, they made super a significant part of the game and its plot, with a visible demonstration of it in the very opening cutscene and a final boss only for him. At some point in development, though, the decision was made to split S3K into two cartridges, which created a weird situation where they'd end up with 14 special stages. They made the decision to create the "super emerald" mechanic, and introduced the Hyper Sonic form, and also gave Sonic's rival in the game, Knuckles, with the same two forms. Also, to justify the previous games not letting Tails go super, they gave him Super Tails only with all 14 emeralds. This turned super forms into a major theme for the game. You'll notice that Sonic's rival, Knuckles, even ends up fighting a Super Mecha Sonic as his final boss, to drive the theming home even harder.
There's a problem to this scheme, though: the whole thing turned into a giant, overcomplicated mess. You'll notice that many people in this thread misuse the terms for S3K's system, too, including the message I'm responding to. It gets FAR worse the instant non-game Sonic shit comes in, like the shows or comics. The massive complexity that resulted from S3K is the main reason why SEGA keeps avoiding any super form from S3K anymore. It's just way too complicated and just results from a quirk of how S3K was released. I'd wager if they hadn't split S3K into two games, only Sonic and Knuckles would have had a super form.
Then why does Knuckles never go super again, you ask? Simple: he's no longer Sonic's rival in the game's overall plot. The only characters they allow to use Sonic's signature ability is the character he's paired up against. In S3K, that character is Knuckles. To use two other simple examples, Shadow got to go super in SA2, and Blaze got to go "super" in Sonic Rush. This is because those games also use the same basic paired character plot structure that Sonic and Knuckles had. After they stop being paired up with Sonic in the plot, they lose the ability for the rest of the franchise. This is why Knuckles doesn't get to go super in Sonic Advance or Sonic Heroes.
Maybe if SRB2 also had a paired character plot structure with a rival, we'd also want to give another character the ability to go super. Knuckles sure isn't his rival here, though, so the justification for why Knuckles can go super in S3K just doesn't apply to SRB2.