99.99+% SLA typically means you get some billing credits for the downtime exceeding 99.99+ availability. So technically do get a "99.99+% SLA", but you don't get 99.99+% availability.

Other circles use "SLO" (where the O stands for objective).

(Anyone know what the details in fly.io SLA are?)

Answering myself, https://fly.io/legal/sla-uptime/ says you get some credits for under 99.9% uptime "provided that Customer reports to Fly.io such failure to meet the Uptime Commitment". So at least currently there's no talk of 99.99%.

You are correct in the legal/technical sense!

Technically, anyone could offer five- or six-nines and just depend on most customers not to claim the credits :-D

Actually hitting/exceeding four nines is still tough.