I’d argue that -splain has graduated from portmanteau part to legit splinter.
From original mansplain to avocadosplain (below), chickensplain, smarmsplain (Mark Hamill), whitesplain, gradsplain, mathsplain, and here linguisticsplain (though I’d prefer lingsplain).
Also: the linguist doing the schooling here isn’t 100% accurate in all these etymologies, but there’s some good examples of analogical change.
It’s also productively extensible in all the ways explain has been. If you want to explain some concept X, you can now write an X-splainer:
In fact, splain is now sufficient as a root on its own, meaning roughly ‘to explain condescendingly’.