"In early 2019, ConcernedApe formed a small development team to continue updating the game. The 1.4 update of the game was developed by ConcernedApe, Arthur Lee (a.k.a. Mr. Podunkian), and Alex Erlandson (a.k.a. zillix). The 1.5 update of the game was developed by ConcernedApe and Arthur Lee. See Stardew Valley for more info about the game’s development. "
So he worked on 1.4 and 1.5 with ConcernedApe. Technically a former Stardew Valley developer, but title is just click bait to get attention. Up to 1.3 was a one man show, and by that point, who could blame ConcernedApe for hiring some help; the game was successful, and had the funds to do it, and help with his vision (possibly with things outside his expertise).
SLaSZT