The bottom line is that GIMP didn't want to be associated with tricking its users into installing borderline malware. If a program's installer is filled with traps that you have to carefully watch for to avoid - that sets off huge blaring alarms in the heads of most experienced users.
This is especially a problem for the open-source community, which still struggles to get a fair comparison with commercial software in the corporate IT world. If even major software gets saddled with nonsense like SourceForge is trying to pull, it could set back progress by years.
SourceForge had better smarten up before it becomes a ghost town. GIMP is certainly not going to be the last high-profile departure if things don't change.
While we're at it, the summary of this story was blatantly whitewashed. Mentioning the parent company link should be a bare minimum, not an excuse to abandon all pretense of impartiality.