I see this as more of a branding choice than a "contribution limiting" choice. It's rather impossible to truly prevent contributions in FOSS (someone can always

I see this as more of a branding choice than a "contribution limiting" choice. It's rather impossible to truly prevent contributions in FOSS (someone can always just make a fork). Instead, this seems to me as a kind of branding where the maintainers are saying that for the upstream they would like to dictate the style the code is developed with including how/if "bugs" are fixed. This seems reasonable to me. Even the best style guides can't convey styles of ideas about code structure, so having this branding of "Upstream is written according to my thought processes that I can't put into words" seems like a pretty reasonable approach for projects maintained by individuals or small teams. Also, in this case the dev is still open to bug reports/feature requests. The only thing they're asking for is to not have their style of thought be disturbed.