It’s always worth looking at the data, rather than the ‘headline’.
Percentage wise, yes BHX grew 129% compared to MANs 70%, but when you look at the absolute numbers, they both grew at roughly the same rate.
So for example, in 2016, MAN had 16672 connecting pax, BHX had 9451. That’s difference of 7221 pax.
Fast forward to 2019, MAN had 28371 connecting pax, BHX had 21654, a difference of 6717.
so as you can see, they both grew at a similar rate, and the difference in absolute passengers is not too dissimilar in 2019 as it was in 2016.
also if we were to take the article at gospel as to where they should have launched, then one could argue MAN-ORD should have launched given it had the highest fares and connections for the MAN figures.
Birmingham is clearly an important destination for Aer Lingus, it would be disingenuous to argue otherwise, but the MAN base was about backfilling the lost Thomas Cook routes and taking a battle to Virgin. BHX has/had neither of these so probably didn’t feature in the plans. (Edinburgh was touted in the initial plans for EI U.K., with Delta serving routes from EDI and being a partner for VS, was probably in the plan to fight the carriers up there too).