Here, I think the majority has the better argument. If someone discriminates against someone based on age, it's almost always going to be based on the general perception that the person is too old, not because the person is too old because he is 40 or older, and wouldn't be too old if he were merely 39. The protected class defines who can recover when treated adversely based on the perception that he is too old and does not require age-motivated bias specifically targeting workers 40 or older.
The dissent also noted that Burnett's supervisor had already decided to fire Burnett before he turned 40. If the supervisor's decision had been enough and didn't need to be approved, then Burnett was arguably fired before he was 40, even if he was notified later. However, it appears that the supervisor needed to consult with HR, so Burnett was not actually fired until a few weeks later after he'd already turned 40.
This blog reflects the views solely of its author. It is not intended, and should not be regarded, as legal advice on how to analyze any particular set of facts.