I have no insight into what happened here, but I definitely have had experience working with and processing the termination of underperformers who don't recognize or accept that they are not meeting the bar and this sounds a bit like one of those cases.
There are probably some aspects of this separation that were imperfectly handled on both sides, but given the context of "guy is fired from company chooses to be interviewed on company site and say the company didn't give him any reason why he was fired" and the company says "you weren't doing your job" - I tend to trust the company.