What this video covers
This is the same bathroom a while later, and the clip is about what changed.
The wall is now affected across its full height, there is growth behind the tile where nobody was looking, and the drywall has to come out rather than be cleaned. None of that happened suddenly. It is the small patch from the earlier video, left alone.
The takeaway
The jump in cost is not the mold spreading across the surface. It is the mold getting behind the surface, at which point cleaning stops being an option and replacement starts.
Read the full written guide: Why a Small Mold Spot Is a Big Problem.