The designation was not a Roman numeral but Lowell's designation for "unknown". It would have been the 9th planet if discovered, at the time Lowell coined it, anyway
Pluto was still considered a planet in the eighties, so at that point an additional planet (if discovered) would have been the 10th planet.
Pluto was discovered in 1930. The term was coined decades before that.
I think it's a semi-deliberate pun. It was Planet 10 when people started looking for it. Then it would be "Planet 9", which is confusing. So they called it then "Planet X" as in 'ex', nerdily harking back to the good old days when Pluto was still a planet.
The theory was proposed and it was anointed 'Planet X' when Pluto was the 9th planet, with X as the Roman numeral for 10. Now it's a play on 'X' being something unknown. Nothing to get annoyed about.
I just bent space from Ix. Many new machines there.