While I knew that its proto name was "Sonic RPG" but I don't think they planned it as a turn based RPG.
Maybe the "Roleplaying" part was because of the adventure fields and upgrades.
While 2006 and Unleashed have hub areas I always felt like SA1's adventure fields felt more "living" and interesting to navigate in.
Very true, adding rpg elements was just kinda the craze (GTA San Andreas and Mega Man X both featured them despite not being the main genre)
And also, the initial devs of Sonic Team came from Phantasy Star, so a little muscle memory and modus operandi likely persisted throughout the games’ history.
On rare occasions, Sonic Team also does enjoy being intentionally a little misleading, as evident by this one April fools post.
Also true on SA1 and Unleashed, their hubs were funnily enough, my favorite part so I’m always baffled some folks celebrated SA2 for removing those.