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.
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.
"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.
That isn't irony. It's at worst pragmatism, and at best not being stupid to continue a lost fight.
They should've worked instead to get proper buy-in from the various parties supporting the current standards to ensure that another GIF incident doesn't happen.
MPEG claimed they were going to create royalty-free profiles of H.264 and then went back on that (possibly due to cunning political moves by Microsoft) and now claim to be evaluating a couple of ways forward for royalty-free codecs (a profile of H.264 again, or building on older MPEG 2 tech). It's fairly obvious that it was pressure from (primarily) Google & Mozilla that put RF codecs back on the table for them.
MPEG-LA reduced the uncertainty around their future codec licence payments, and as a result their maximum profits. Again almost certainly due to Google and Mozilla.
They didn't take their ball and go home, they negotiated and competed and as a result won valuable concessions that continue to have value as the next round of codec development begins.
They fought 2 years for getting patent-free video recognized, probably at the cost of some of their userbase, and a lot of anger from web-developers who saw their lives complicated.
Today, they seem to be reconsidering their position because they think it's a lost fight.
How is this a blow to their credibility? What should they have done? Instantly bend over next time? Stubbornly refuse to reconsider and lose the best hope for a truly free phone OS because nobody would want to use it?
If you lose a war, it's never pretty. Kudos to Mozilla for taking a hard stance against patented standards for as long as they could. They deserve credit for that.