In the specific case of Celeste, the game was playable before development, because it was preceded by the PICO-8 version.

Valve did this a couple times by just buying out teams who made mods or interesting games - Narbacular Drop becoming Portal, that goo thing becoming the goo thing in Portal 2, Counterstrike becoming Counterstrike, then a CS mod becoming Left 4 Dead.

"Every big program that works evolved from a small program that worked" is even true for games