Given that they registered futurestack.io it seems almost certain they did check this. .io is nice and quirky and all but if .com was available you can be fairly sure they'd have gone for that either instead of or as well as the .io domain.

It's ideal to get those, but for a good name it's not always required.

I challenge you to come up with a plausible name for a conference series that has an available twitter handle and dot-com domain.