I have my doubts about this rumor. Moreso releasing the same day as the other versions.
GTA 6 is one of those games where Rockstar wants to perfect it the best they can and utilize the most graphical features they can. I highly doubt Rockstar would be potentially willing to potential hinder or scale back on their vision by trying to keep the Switch 2 version in mind. Especially on a platform that was never known for GTA and a company they've historically neglected releasing GTA games on.
Also, I would believe this rumor more if we saw a bit more Rockstar stuff on Switch up to this point. Rockstar definitely put more effort on Switch 1 then they did with any previous Nintendo console releasing stuff like LA Noire, Red Dead 1, and of course the GTA trilogy. But still no GTA 5 yet or even Red Dead 2 yet. I feel like if GTA 6 were coming, they would've at least released GTA 5 first on Switch to show full interest in the Switch, but they haven't. I feel like it'll be a really aggressive jump to not at all release GTA 5 on Switch after 9 years but go straight to dropping GTA 6 on the same day as other platforms.
And just from a logistics standpoint I find it difficult for Rockstar to be able to release on the same day as other platforms. Rockstar doesn't at all need the Switch 2 for GTA6 to be a breakout success, they wouldn't delay the game or try to adjust their schedule just for the Switch 2 version. This is why I doubt this rumor.
In terms of Switch 2 handling GTA 6. I dont think it'll be as hard as some people imagine actually. The RE9 port really impressed me looking nearly exactly the same as the other platforms. Yea RE9 is definitely way smaller scale but I feel like this gen especially is showing how scalable games are even putting on weaker hardware. The PS4 still holds impressively well to the PS5 and the Switch 2 is more capable than a PS4. So I think with optimization its possible, but logistically speaking I don't think Rockstar has shown enough signs to be willing to do it, let alone release it the same day.