Because it either would have been wildly popular, and there would be a lot of others, or it would have been easier for Drizzly to to discover from a popular startup blog than from a not so popular stock artwork. Perhaps they could have carried out a search found the stock source as justification for copying it, but that wouldn't have the same story of independent discovery.

Also if you take asmartbear at his word, that sort of implies he at least somewhat believes his designer :) If he didn't think his designer had any scruples about saying that they made that, I don't think he'd have said it so plainly, as "was made by a freelance designer"