Welcome, there!
My not-based-in-anything-but-overthinking thoughts include...
- Looking back, this May tour just passed was announced about 10 weeks prior. (2010's w/ ~7 months notice, but that included a festival date.). 09's Manchester show was announced w/ ~10 weeks notice, the London w/ just ~8 weeks notice. These US South dates have only just been announced for January.
- As a one-off show with other parties involved it makes sense it would be organised earlier than a regular gig. Also, the Barbican's kinda a big place to sell out (and tMG don't always sell out here (I think only 1 or 2 dates of the May tour sold out?)), so w/ the Barbican curating this A4/tMG thinguh - the B. might want it to sell a bit first?
If we guess 10 weeks's normal notice, and
if we'll be graced with more to come, it wouldn't seem particularly out of sorts to not hear of it 'til the new year?
midgoat