TYPO3 Core

Thanks for reaching out. Although I will reject this bug report, I'll try to give some information about why.

I'm kinda puzzled why this class is annotated for internal use

This class is marked internal because it abstracts the way TYPO3 deals with class loading, when not being install with composer.

Right now we decided to just ignore the warning

Fine. Just be aware that it still isn't a good idea and any code / concept relying on this API being called will NOT work any more when TYPO3 is composer managed (installed with composer, aka composer mode).
In composer mode all class loading is delegated to composer and while calling this API will write some files, these files will never be included in the TYPO3 bootstrap.

It seems odd anyway, as the method itself is actually scoped 'public'

Nothing is odd about this fact. TYPO3 code needs to call it from several places. But third party extensions must not call it. End of story :)

Another suggestion which'd solve our particular case

I can't go into much detail here (I don't know why you did things and also not in detail how), but you should think about solving your use case differently.

I would start questioning that generating model classes is necessary.
If it still is, there are several possibilities to get things working without calling this code.

1. Register your own autoloader
2. Write the generated classes into an extension folder and register class loading for this folder in composer.json/ ext_emconf.php

If you need more support regarding this, feel free to ask on Stackoverflow and/ or in our community Slack channels.