This is a good question to ask. It can't be, for the reasons you guessed.
This is not the first time this sort of thing has happened. When Pluto was found by Clyde Tombaugh he was looking for Planet Nine, which Percival Lowell had calculated must be present based on the orbits of the outer planets. But it was quickly realized that Pluto was too small and in the wrong orbit for it be Lowell's deduced planet. (And even then they worked with a too high estimate of Pluto's mass, it wasn't until the 1978 discovery of Charon that we got a good estimate of Pluto's mass. It is hard to get a good mass estimate without something else in orbit around it.)
The Pioneer and Voyager missions gave us much better estimates of the masses of the gas giants, and my understanding is that if you go back and redo Lowell's calculations with those correct masses, his planet disappears. That's my best guess as to Planet X, that our constants are wrong in some way, but we'll see.
One of the other theories for Planet X I believe has been debunked as absense of evidence. There are gaps in the documented bodies orbiting the sun that could imply an object clearing orbits, but they were dismissed instead as sampling errors - there are parts of the sky that are easier to catalog than others, and so of course we have cataloged the easy parts more thoroughly. We need observation stations in a sun orbit to see the parts we can’t see easily from an earth orbit.
It is a very odd orbit. Obviously it doesn't match the expected mass, but the orbit makes you wonder what else might be out there. As someone else posted:
