PySide6 -> Qt6, PySide2 ->Qt5, so PySide6 follow EOL dates for Qt6, not Qt5.
It's the first time I see that website, and I have no idea if those dates are official or accurate.
Also, I think it's important to make a difference between having the source code or having binaries.
The 5.15.x source code is been released according to the KDE Free Qt Foundation, we recently published the 5.15.3 source files in https://download.qt.io/official_releases/QtForPython/pyside2/PySide2-5.15.3-src/ for example. The same will happens with all the future versions (so far 5.15.9 is available for commercial users).
If you are considering 'wheels', then yeah, 5.15 will not get any other new OSS wheels, and the moment Qt stop publishing versions, wheels will stop as well.