You know, here Blizzard claim, that promotion was planned long time ago, so it's clear indication, that it's not tied to current loss of subs. But... What if Blizzard simply knew in advance, that xpack was going to be bad? You know, they actually had Alpha/Beta feedback and they actually experienced enough to know, whether game, they develop, is good or bad.
UPD
Why it's tinfoil, if it's obvious thing? If you'd dig this forum, you'd probably find some old threads, where we warned you, that datamined half-year sub meant, that xpack was obviously going to fail. Blizzard are experienced enough to realize, that they're making filler xpack. Especially, if it's their initial plan. You know, they have been developing this game for 14 years. Ask yourself, how can they make exactly the same mistakes again, again and again then?
UPD2
It's also right moment to implement such promotion now, cuz all players, who have bought BFA + 1 month of sub, have their sub lapsed by now.