Source is KeplerL2, who is generally considered a reliable source for insider hardware info, particularly on AMD GPU hardware and AMD SoC for consoles.
Previously I would have personally estimated Steam Deck 2 to release mid 2026-early 2027, but the recent info about an upcoming Steam Machine made me think that maybe I should push back that estimate.
Of course even if we assume this is reliable insider info, a lot can change in couple years, so things can definitely change.
Just a note, Proton is mostly Wine, a program that has been in the making for decades and could be used to play Windows programs (including games) on Linux long before Valve started pouring resources into it. They have “just” concentrated development on games.