Asa Dotzler is community coordinator for Mozilla, I think he knows what he's talking about...
My interpretation is that 16 months ago, Firefox stopped caring about Acid3 because the remaining points were disagreements on how to interpret the standard. Not because they were too lazy to fix it, because they had a point.
The fact that it has been resolved only now is irrelevant. If you take a Firefox 16 months old and run Acid3 on it now, it would score 100% - I think that's what he meant.