I was referring to the "HTTP request in Tokyo to find the nearest instance in Sydney" part which felt to me like a differentiating feature- no other cloud provider seems to have bidding or HTTP request level cross regional lookup or whatever.

The "decision that long predates Corrosion" is precisely the point I was trying to make - was it made too soon before understanding the ramifications and/or having a validated technical solution ready? IOW maybe the feature requiring the problem solution could have come later? (I don't know much about fly.io and its features, so apologies if some of this is unclear/wrongly assumes things.)