"How do you know the hypothetical person is a "cis man" - because I was responding to a specific part in Devon's text where he says "cis guy".

"By it's very nature, 'cisgender man' implies conformation to and a strong degree of comfort with current societal expectations associated with your physical sex." - no it doesn't, you can be a nonconforming cisgender man easily. Cis simply means that you identify as the gender you were assigned at birth, which in the case of "cis man" means if you were assigned male at birth and you identify as male still, then you're a cis man.

It has nothing to do with whether you conform to societal expectations or not. You can be a flamboyant effeminate man and still be cis. Plenty of men do that. For example David Bowie did that for quite some time, wearing dresses and having long hair. He was a very nonconformist man but he was a cis man nonetheless.

"In theory, you would expect masculinity from male persons and femininity from female persons" - and yet somehow masculine women exist, and feminine men exist.

"How do you know whether or not anyone in particular is rejecting masculinity (or femininity) as a whole?" That's why you ask them rather than assume they are.