Why would anyone try to register via a non-official app first (especially for a procedure like signin-up) is beyond me.
You may or may not have heard this before, but the app is not the instance is not the platform. I registered both my Mastodon account and this Lemmy account via their respective instance websites. I used mastodon in the browser for literally over a year before installing an app for it on my phone.
Apps are alternative front-ends to the fediverse, even "official" ones.
"Basic stuff" is very weird to read for me when many of the internet services I have accounts for don't have apps - and I would rather they never make an app for it. My electricity bills, my hosting costs, my home internet, all are done through web pages that I can access from any internet-connected device, unlike an app.
Not to mention I appreciate being able to type things on a bigger screen and physical keyboard when I register for things.
Lastly, it is much easier for me to deal with a sloppily made website than a sloppily made app. I can use extensions, and if need be can open up the network tab to see if the registration request was accepted or not before the website malfunctioned on my end.