Why do you say truncate is non-transactional? Something simple proves that it's not?
Or just read the documentation for the current version (I seem to recall it used to be non-transactional, maybe...doesn't matter now).
Sequences really shouldn't have been a surprise given the great lengths we go to document their gap-ful nature and this property.
Most anything a typical user is going to do within a SQL transaction is going to either be transactional or it will be disallowed to execute said command within a transaction.