Which is the level he's acknowledging it on. Short term profit that cannibalises product value and user goodwill is all-too-common in the modern corporate climate, and he's acknowledging the elephant in the room.
> ...but ignores how shitty that would contribute to making the internet
Presumably, that would be the reason "he considers it "off-mission""
While I agree that him phrasing his reason not to so weakly instead of "doing so would kill firefox" is a little concerning, a CEO probably doesn't want to be overly honest about the other, less investor-friendly elephant in the room, "the only reason anyone uses Firefox is for uBO".
But also, we don't actually know how exactly he said it, since it's not a direct quote. For all we know, it was an offhanded remark, or he said it in a tone that meant he knew what a terrible idea it was. We're trying to read tea-leaves from a single paraphrased remark.
And believe it or not, human behavior is such that something that is not even i in the space of possibilities is much less likely to occur that something that has been considered and rejected. It might have been rejected ow, but what the calculus changes?