I can't be absolutly certain, of course, but given the quality of the work itself I can tell that the people who did it aren't amatures. I also know the probability an accomplished, professional designer claiming non-payment when payment was made is virtually zero.

Indeed, having worked in related fields for the better part of two decades, I cannot think of a single example where a designer attempted to shake down a client with a bogus claim of non-payment. But I can think of an astonishingly depressing number of cases where clients wanting something for nothing decided that stiffing their vendors was an easy and acceptable way to increase their own margins.

All other considerations aside, if the client didn't pay, the work isn't theirs. By law. Full stop. And if the client was stupid enough to host their site with a vendor they're shafting, the the vendor is well within their rights to limit access to that site in the fashion they did.

That'd turn this from "payment dispute" into "extortion", wouldn't it? A lot to risk to extract a few grand from a client who already paid you...