At a basic level, methods of combining oligos to produce long strands have been known for ages. The challenge is to be able to produce them with low enough error, high enough yield, and enough freedom on sequence. Low error improves your yield, reduces the amount of purification and amplification needed, and lets you make longer strands. Sequence constraints can be significant, too, especially around repeats.

If you're talking about Twist's gene fragment product, they advertise that as maxing out at 5 kb. Most, if not essentially all, of that month delivery time is likely the combination, not the oligo pool production. I think the Sidewinder people are actually using Twist pools; they're doing up to 12.5 kb.

By comparison, we recently needed something in the 20 kb range, with a not-so-great sequence, and it was a multi-month process to have a company produce it.