I don't think that his statement regards to the "quality" (in terms of quality assurance capabilities eg.) of the users itself.
What the invity only approach brings is, that they are able to upgrade the backend (servers, databases, bandwidth for example) in babysteps. This may lead to a better user experience because downtimes are lower, the site itself is more responsive, bugs can be fixed faster due to lesser impacts of new software rollouts etc.

So the site itself is (or may be because we don't know ) of higher quality if there are less users allowed to access the site in early stages of the development. Anyways, its easy to get an invite link and if it helps them to deliver a stable and working product to the masses I am fine with it as long as they don't extend the invite only phase for too long.