They could ban him "permanent suspension" (that is provided for by Facebook Rules) as are things like temporary suspension (for a specified period rather than indefinite), deleting offending posts. The issue is about "indefinite suspensions" which don't exist meaning Facebook have applied something other than their rules.
They could change their rules to include "indefinite suspensions" but all that sort of stuff is the responsibility of Facebook rather than the Oversight Board who were "ruling" (or rather not ruling) today.
So I believe a 50 year suspension would comply with Facebook's current rules and that would allow the Oversight Board to then make a decision about upholding or quashing that punishment.
Ian