Hi,
If you have set a compliance policy to allow only iOS 13.5 + to access your email system, it is then independent from the application used to access it.
The compliance rule should block any device below iOS 13.5 to access the mails, wether they're trying to reach out from the native mail app or Boxer or any other mail client.
To answer your question, from my understanding, devices that cannot be updated to iOS 13.5 will not be able to connect to the mail through Boxer or Native mail.