It doesn't undermine anything I said, because I was not stating that "ultimogeniture is feudalism", or that the specific forms I mentioned - and notice I said a modified form, with caveats - of ultimogeniture were in use, in response to you asserting that there is no inheritance except gavelkind in feudal society.
And, bear with me here, the ultimogeniture forms I mentioned may have been partible, but they weren't gavelkind. Gavelkind is a specific inheritance form with specific laws. So specific, that it doesn't appear to have been the accepted form for even the whole of historic Kent, much less the rest of England, and even less the whole of the feudal model.