I wonder how this modifies their StackExchange offering. Does this mean that if you are doing anything beyond a internal knowledge base Stack Overflow is now a competitor?
If a site gets to the point where it is a substantial business, they may well want to partner with Stackoverflow more closely anyway to get features, advice, access to whatever revenue generating plan they come up with.
Either you get a successful site in that vertical, or somebody else pays you $50k/year for the right to have it instead.