Barrier Gate Five Seal Technique: Eight Gate Lock-up:
"but any being attempting to pass through in either direction will be destroyed by unquenchable flames."
Fūka Hōin - An advanced fūinjutsu that, upon utilization, absorbs any and all flame -- even extremely powerful fire such as the Amaterasu -- into its depths
So they have a fuinjutsu that can seal amaterasu the top level fire technique, but this barrier is all of a sudden untouchable? I don't think so.
Use Fuka Hoin on the barrier. Problem solved.
But in all seriousness that 'barrier' isn't even supposed to be allowed anywhere. Auto-voided if more people knew about it. So void it here if you want. It's BS.