This is true, but it is too simplified.

You can easily have differing 2n in even mammals. All it requires is fusion of only the X fragment but not the Y chromosome.

See the Indian muntjac: https://www.jstage.jst.go.jp/article/cytologia/70/1/70_1_71/_pdf 2n=6/7

Which funnily enough can actually breed with the Chinese 2n=46

Additionally plenty of chromosomes do weird fusion and duplication stuff, meaning you can actually have specific chromosomes that can be dropped with no I’ll effect.

There‘s also insects with males having a single chromosome less than the females.

But that should only possible with ZW/ZZ insects, and not our XX/XY relatives.

And it’s always the case for insects completely lacking Y chromosomes. Take for example stick insects. The females have XX chromosomes the males just have X0 chromosomes.

Like they lack the second sex chromosome in regular existence.

But really as I said: if you have chromosomes that can be dropped wirh no Ill effect, uneven is possible.

And since males are the genetically inferior variant of the ancestral asexual progenitors, usually it‘s the males that will have uneven chromosomes, because their Y chromosome can in many cases do whatever and still leave a fully viable animal.