Not sure what they’re referring to, but there are a number of warnings when you install an APK and you have to enable a setting to allow you to install an APK. It’s not challenging or restrictive, but a user who is unfamiliar might be discouraged from installing an app this way after seeing the warnings.
You can do t-shirts for $10 with even smaller runs than that. I’ve done batches of 50 or less at $10 a pop going with a local screenprinter. This is usually 1 or 2 colors.