Any evidence of those backsoors?

This reads like “your front door lock is the perfect place for security services to have a master key.” True, but not strong as an argument against having a lock.

>This reads like “your front door lock is the perfect place for security services to have a master key

Even that analogy fails because MTE is in addition to existing countermeasures against memory corruption attacks. In the worst case, compromising MTE just means you don't have MTE, not that you get arbitrary code execution.