Yeah, Baldur's gate 3 is definitely not a big threat to Starfield. Yes, Baldur's Gate 3 sold 2m+ on Steam Early Access so far, but lifetime it will probably be less than 6m on Steam. The last 2 Bethesda singleplayer RPG's, Skyrim and Fallout 4, sold more than that on Steam, as of the 2018 Steam data leak Skyrim was at 13m while Fallout 4 was at 6.6m, now in 2023 it is probably 15m+ for Skyrim and 9m+ for Falllout 4 I'd guess.
Meanwhile Larian games, like all isometric RPG's, sell considerably less on console than on PC. For instance Divinity Original Sin 1 seems to have sold somewhere between 1.5-2m copies on PC, compared to about 500k on PS4 and 250k on Xbox One based on old VGC physical data plus my own digital estimates. Bethesda RPG's on the other hand sell considerably more on console, Fallout 4 likely sold 10m+ on PS4 and 8m+ on Xbox One lifetime, while Skyrim seems to have sold 9m+ on 360, 6m+ on PS3, 6m+ on PS4/PS5, and 4m+ on Xbox One/Xbox Series, once again based on old VGC physical data plus my own digital estimates.
No matter what way you look at it, September is definitely a more strategic release window for Starfield than the previously rumored June window. June has 3 other AAA releases, 2 of them RPG's (Diablo 4 and FF16), and Diablo 4 and FF16 will both almost certainly be higher selling RPG's than Baldur's Gate 3 will be. By comparison, Starfield is currently the only AAA release scheduled for September. On top of that, Starfield releasing in September is more strategic for hardware sales, gives them 3 more months to fix the current production issues that are plaguing Xbox Series X but not PS5, as well as extra months to save up units for likely Starfield Series S and X standard bundles and limited edition custom consoles.