I realize this is more about the X-men universe, but the statement "mutants aren't human" bugs me.

The human body accumulates on the order of 10 quadrillion uncorrected mutations per day, supposedly:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC47258/

(I'm assuming 30 trillion cells and that ~2.7% of mutations go uncorrected based on the rat figures mentioned)