Giving Access to Apex classes Through ANT

Use namespace__ApexClass as the apexClass format in PermissionSetApexClassAccess for Apex Classes belonging to a managed package or in a namespaced org. (Rather than namespace.ApexClass used in all other managed Apex references.)

The only documentation I can find regarding PermissionSetApexClassAccess says the following which isn't helpful.

apexClass: Required. The Apex class name.

I only discovered the correct format by installing a package in a scratch org, created a Permission Set in the UI with access to a managed Apex Class, then pulled the source down to inspect. It feels bizarre that PermissionSetApexClassAccess requires double underscore.