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it seems like they're also doing some sort of ad hoc distribution for those without dev tools: https://testflightapp.com/join/25bf5108ebf60dd935baa57f36856...

Testflight is generally limited to 100 people/devices (as that's the limit Apple gives a developer account)

100 Per Developer, and now it's open source, meaning any developer can package it distribute it via this method.

you will need more than a single testflight link for that distribution scheme to work.