Because he's not saying "we tried, but it didn't work out in the end". He's saying there is no justification not to use available platform decoders. Yet they were not shy to provide them in the past. He's not addressing why those are no longer relevant arguments.
Obviously the real reason they're doing this now is because on their new mobile platform, the Gecko runtime is all there is, and without it supporting H.264 playback, the platform would be considered incomplete at launch more so than their desktop browser is for not supporting it. I get the motivation. I'm not saying it's wrong - I don't agree with it, but that's not the point here. The point is how bad it makes Mozilla look to change their stance so willy-nilly.
Yet they were not shy to provide them in the past.
The reasons are still there - it's even pointed out in the parent discussion that they're still a problem on Windows. One main difference is that Windows 7 is an order of magnitude more widespread now than it was 2 years ago - when those posts you refer to were written.
And as you already admitted - they're doing this on mobile first. There's no guarantee yet it turns out to be actually feasible on desktop if those same arguments can't be properly addressed.
But the Windows problems aren't the only justifications they published back then. In the second linked post O'Callahan writes:
"It pushes the software freedom issues from the browser (where we have leverage to possibly change the codec situation) to the platform (where there is no such leverage). You still can't have a completely free software Web client stack."
Given that it strikes me as ironic that Mozilla is now building a platform - i.e. a platform where they definitely have leverage - that is going to support H.264.