Where can we learn about this? As far as it seems, qt is still, as always, distributed under the LGPL; which is as "open source" as it gets. The main site of qt even lists explicitly the Four Freedoms of the FSF : https://www.qt.io/licensing/open-source-lgpl-obligations
But the Qt Company has tried to make using the open source releases more and more inconvenient: licensing terms for the commercial version that are not at all forthcoming to OSS-to-commercial converts (implying that you need to pay for commercial licenses from the start), more limited access to precompiled binaries and registration obligations when you want to use the official Qt installer. That behavior seems to say that the company is bound by an agreement that they honor only reluctantly.
(To put a finer point on the specifics you mentioned: I believe they once had a policy of making the QML-to-C++ compiler available only to commercial customers, and not to FOSS users, treating it as something distinct from the 'core of Qt'. They later changed their minds on this. [0][1])
Presumably they could start work on a whole new toolkit, call it something other than Qt, and be free of the KDE agreement. This would no doubt be much more work than a typical major Qt release, but they do presumably have the option.
[0] https://www.qt.io/blog/the-new-qt-quick-compiler-is-coming-i...
[1] https://stackoverflow.com/questions/9448296/is-qml-translate...
Which part is not true? It all looks correct to me.