I also heard Valve say “it’s just a PC”, does that mean it’s suitable for software development too?

Yes, I used my Steam Deck for software development briefly. But don't use the flatpak versions of the IDEs, use the tarballs instead. The flatpak sandbox will cause weird issues when the IDE is trying to access resources outside its sandbox. Also keep everything -as much as possible- in your home directory as intended by SteamOS, don't try to unlock the read-only filesystem, even though you can, you will lose everything when SteamOS updates.