Allow me for a moment to focus on the "if " in that comment.
Remember we are making decisions on what will be in the future, not as things are right now.
Especially given that all the stuff being talked about here won't go truly live for quite a while, I suspect min six months is not unreasonable, what is it that demands continuing support for qt5? Sorry, I just don't know the answer. If it is truly necessary I understand (there is much I don't know), but ...
From what I'm seeing, therre is a LOT of specific (literal) shiboken2, pyside2, qt5 stuff in the codebase. I'm still finding them. There is even a bunch of pre qt5 stuff (even though 4 was killed off) through conditionals. Perhaps a lot of that is due to poor implementation (specificity). The effort to support 5 AND 6 simultaneously will, I suspect be huge and complicated, and take a long time to implement.
If 5 support is a 1, and 6 support is a 1, what is the cost of supporting both? I suspect more like 5. But this is just a suspicion.
What is the cost/benefit tradeoff of keeping Qt5 while supporting Qt6?