Given the litany of changes, differences, and divergences, it's really not. Half the cast disappeared, the levels are nearly completely different, Maria's backstory is totally changed, and even the overall story of the game is significantly different enough that you couldn't swap one for the other on the main timeline and still have it work 100%.
Castlevania: Dracula X is a separate game made along the same design guidelines as Rondo, and one whose dev team came to their own conclusions about how to meet those guidelines. There's shared assets between them, but calling it a port because it shares assets is a claim that's honestly barely firmer than calling Harmony of Dissonance a port of Symphony of the Night because they share some map structure here and there and Harmony reuses sprites from Symphony. Reused assets do not a port make, imo. If it did, we'd probably have to start calling Harmony of Despair a port of the first Castlevania as well as Getsu Fuuma considering it has DLC maps that lift the ENTIRE level structure, art style, bosses, music, and playable characters from those games; and I think the press would acknowledge that's a pretty silly claim to be making (but it does highlight that Despair is more a genuine "port" than CDX, imo, even if my mind vomits a little during that logical leap because holy shit even Evel Knievel could only barely land that).
CDX is it's own thing, and I'm tired of the press constantly trying to call it a port. Symphony Saturn is a port: an effort to take an existing game as it was and put it on new hardware as best as possible. There's new or changed content here and there, but nothing that fundamentally alters the overall game or plot. Dracula X spins new material from whole cloth on almost every level -- that ain't a port, no matter how much the gaming press' insistent terminology tries to make it one.
Sorry. Had to get that off my chest.