I'd consider the original comment a positive contribution. Just because it can be seen as having a negative tone, this one of caution, it is backed up with a heft of substance explaining the thought process behind it. It also resulted in another informative response from the founder. That's a good thing we should encourage.

A negative comment is one that doesn't contribute anything. A negative generalization about a community that doesn't provide an explanation could be seen as an example of a negative comment.