> If I were to acquire a commercial licence I have to get the source; that's the GPL.
> If I then sell an application built with that code I have to provide my customers with the source and a copy of the GPL. That's also part of the GPL
Just to clarify, Qt dual-licenses this code. There is a license for commercial use, and an OSS license. The OSS license in play is LGPLv3 (except for certain tools which are GPL). They have a FAQ about this: https://www.qt.io/faq/
If you're choosing the commercial license, then there is no requirement to provide source.
> In that case I fail to see how anyone who wants a copy of the supposedly commercial-only LTS version can't get a copy,legally, from a customer of a commercial licensee
As near as I can tell, there is no special "commercial" code as such. Anyone can get a copy of the source directly from Qt. The commercial license gives you additional tools and support as well as LTS binary releases.
Because Qt is using a mix of different licenses for different parts, the licensing situation is quite confusing and I may have some details wrong. If you're intending to use Qt in a commercial product, it would probably be a good idea to get an actual legal opinion.