Indeed, the W3C's work on HTML5 is irrelevant. It exists only for the purpose of giving the W3C-qua-organization the appearance of being involved in the further development of HTML.
They have different purposes. In practical terms, the "HTML5 standard" is really defined by whatever the most popular browsers implement - we saw this in vivid detail when IE6 was the most popular browser and Microsoft made a mockery of the standard. The WHATWG standard exists to track the evolving consensus, so that the major browsers don't diverge too much and we don't go back to the web c. 2005. The W3C spec is so that other interested parties, ones who need to have a finalized doc to shoot for, have something to shoot for that everybody has agreed upon.
I think that's disingenuous — even as someone who has been around the WHATWG for longer than the W3C has been working on HTML5. One clear reason that publication as a REC is important because it implies a royalty-free patent grant between all members of the WG.
I haven't been following the situation super-closely, but I understand that the threat of W3C not ratifying the HTML5 spec as it is (or was) has curtailed the behaviour of certain individuals on a number of occasions during the HTML5 process. If so then the W3C's involvement with HTML5 has not been at all inconsequential, even if you view the formal announcement today as unimportant.