Quote:
the guy is talking about LP cartridges(needles-stylus)..is this true?

The medium itself is physically limited at the extreme low end, sure. But needles, even really really good ones, have reduced separation in high frequencies. This isn't necessarily an undesirable "problem" either, depending on what you're trying to do, or listen to.

Quote: the separation specs are always lower at 10kHz than they are at 2kHz

Yes, it is almost always true, both statements.

Consider, at low frequencies, when the frequency is low enough that the arm can actually move back and forth, you won't get much cartridge deflection. Not a big issue with most, but true, and also bear in mind bass is limited in amplitude by the cutting width vs. time on record, as well as the cartridges' tracking ability.

At high frequencies, stylus beam resonance almsot always reduces the channel separation. It's probably impossible to make a stylus beam symmetric, etc, enough to not have that problem. The question is generally "how much" not "does it happen".

I'm not aware of a cartridge that doesn't do this. On the other hand, testing is iffy because it's just as hard to cut a lacquer that doesn't also have the problem.