Considering that they're the guys making "Dota 2", I'm inclined to agree.
(Well, he's "the person of Dota", at least. Mr. Ismail is far too reclusive to ever show his face.)
Unfortunately, it pains me to say this, but this is the truth: If you tried to commercially release your own DOTA, and if you'd copied skills/heroes/items from the original game, Valve, or rather Ismail, would sic lawyers on you immediately. I know this because they tried to do it to S2, the creators of HoN (a very similar game).
So it's unfortunately not true "There really is no rights holder to the game". Valve/Icefrog hold all the cards at this point.
It's hard to explain. You could only really understand if you'd played the game. "DOTA" roughly means "A highly/specifically competitive game, wherein that game also includes specific heroes and items which competitive players have spent years practicing with and the community in general have come to expect". For example both DOTA and HoN have an item which grants you 10 seconds of magic immunity on use, and can't be used again for more than 1 minute. So if your game doesn't, then it's probably not "a DOTA".
HoN and DOTA both share about ~75% of the "details" (and almost 100% of the details that matter) so it's very much "a DOTA". Whereas League of Legends shares maybe 5%.
A fine analogy, I think, is Linux kernel. Would you not give Linus credit for being in charge (accepting/rejecting patches etc) eventhough he admitted more than once that he doesn't do much nowadays; everything is mostly from the community.
Probably because the claim he invented DotA is wrong. I'm not trying to discredit him, he's probably the single largest single contributor to the genre. But saying he invented it is wrong and a smack in the face to those who did.