This is the weirdest part of Qt licensing for me. An IDE is an IDE, you should not care what I develop with it. I don't even understand how is that possible, since Qt Creator is GPL3.
very simply because it isn't possible and the article is just wrong. You can entirely use LGPL Qt Creator to develop proprietary apps. What you mustn't do is e.g. ship a modified version of Qt Creator as part of for instance an embedded SDK and not ship its source.
And probably this is done to be able to sell the IDEs, maybe based on n. of users, etc. Never bought Qt so I don't know how they license "users".
This is btw not the only trap in the license agreement; if you sign that, you are significantly less well off than if you can do everything with LGPL.
Because doing so would be violating the terms of the commercial Qt license you paid for.