Depends on who you ask, and also the context in which you're asking. I think the people who say this don't really realise the implications of claiming that PLL parity isn't truly "parity" – as you've noticed, it leads to the weird conclusion of swapping two corners not really being "parity" either.For 4 by 4, PLL parity is considered not to be real parity
In the context of 4BLD, it's reasonable to consider PLL parity to not be a thing, since you're directly solving the edge pieces rather than doing edge pairing. Instead, there's corner parity: when you have an odd number of corner targets. Your example of swapping two corners falls into this category, but so does something like a T perm (or even more simply, a single U move).
In the context of a reduction method (or a variant thereof, like Yau or Hoya), there are two parity problems: one where the parity of the number of bad edges is wrong (aka "OLL parity"), and one where the permutation parity of the corners and edge pairs is wrong (aka "PLL parity"). The former type of parity can also be explained in terms of the permutation parity of the individual edge pieces, but you can't break down the definition of "PLL parity" any further (*). This tends to confuse people who think that parity can only apply to a single piece type, or only to single pieces. (You can blow their minds by handing them a 6×6×6 supercube with two pairs of oblique centres swapped. It's not solvable.)
(*) Not for the usual speedsolving methods anyway, but in the computer-solving method Three-Phase Reduction, it is possible to describe PLL parity in terms of single pieces (although it still involves two piece types). In the second reduction phase, the parity of the edge pieces is solved, so OLL parity can never happen; the second phase also splits the 24 edge pieces into two orbits of 12 pieces each. In the third phase, edge pairing is done, while also ensuring that the parity of the corners and one of the edge orbits (doesn't matter which) is solved, which ensures that PLL parity can never happen.