Personally I disagree with the statement, first off, I don't see an alternative explanation offered. the point is an easy analogy to give them rough concepts. looking at the problems listed in the OP.

Gmail users believe the only ways to access a Gmail account are through the official web client at mail.google.com, and the official Gmail app for iOS and Android.

First off there... so the web client off the bat.... what's the problem there, that we aren't burrying them with "oh if you like you can use alexandrite, or one of 30 other web clients, and then tell it the instance". The point is we're trying to reach out to the non tech savy. If their assumption gets them to something that works, then there isn't a problem, just as not knowing that they can install an e-mail client to check their gmail, isn't stopping them from using gmail.

Now the andriod/ios clients, that is the one drawback, you do have to tell them the name of one of the apps, and tell them to pick the website they made their account on from the dropdown. It's not a huge deal but it is an extra step. If the goal is to reach out to the non tech savy though, the goal has to be to minimize the steps as much as we can.

Then it goes on to say people are picking instances based on moderation politics etc... Lets face it regular people don't... and they don't care. Really like 2% of people actually hit points where moderation is a visible thing to them. usually because they are on the edge of a political side.