As I've been saying for a while now, this isn't true, at least not in the case of many third parties. I was posting on GAF back when we still got the complete NPD numbers, and with very rare exceptions, multiplatform titles not only sold worst on GC, but
disproportionately so relative to the platform's userbase. It's why, for instance,
Sega Sports completely abandoned GC in early 2003, because their first two years of sports titles - a key genre for what we'd today call the "dudebro" crowd, obviously - sold so badly on GC compared to even the Xbox versions.
Yes, there were a few key third parties like Rockstar that barely supported GC to begin with, and others that Nintendo alienated with its refusal to support online on the platform, but the ball has always been primarily in Nintendo's court, and any difficulties they currently might be having in getting third parties behind the system are a result primarily of Nintendo's own past decisions. I don't believe that purely irrational contempt for Nintendo is a significant factor here, even if it sometimes come off that way.