Eh, that's an excuse.
Dude had the dream of catching Mew ever since the Summer camp
He also had no idea that Project Mew was a thing until it was mentioned to him.. and the opportunity was only offered to him due to the fact that he’d captured Suicune, deeming him worthy. Ergo, Suicune was the catalyst for Goh joining Project Mew.
Come to think of it, what did Horace do to be deemed worthy? Gary I get, due to his sheer amount of dex-registered Pokémon and fossil research, but Horace? He’s literally just interested in Celebi as far as we know, that’s all..