Ok guys I did some further testing.
As long as you have at least one app from pgyer installed on your phone which you have previously approved to run, additional apps from the same source will run on your iphone without iOS asking you to approve it because pgyer will be regarded as a trusted source.
Once you uninstall all the apps downloaded there, it will ask you again when you install one.
So it seems to me, as long as you have this WhatsApp Beta installed on your phone, pgyer (iPhone Distribution: Human Langxiong Advertising Decoration Engineering Co, Ltd.") will be regarded as a trusted source.
So if a malicious app with the same signature were to be installed on your phone one way or another, it would be able to bypass the iOS security check with no problems.
So be careful when you install the Beta, I am not sure if the trade-off between have a high res version of whats app and possibly introducing a severe security issue onto your phone is worth it.
If you use your phone for anything else but strictly personal use, I would advise against installing anything from pgyer.com