In that discussion (and indeed in the message you quote), the mozilla developers had every incentive to say, if it were true, « we have decided not to replace fennec with fenix on Android until fenix supports ad-blockers ».
They very visibly didn't take the opportunity to do so.
From what I can make out on github, they're planning to replace fennec with fenix around the time the next ESR comes out (which I think is early 2020), and there isn't currently a project in progress to add the necessary web-extension support to fenix.
