Before I'd ever heard of MGTOW, I had a similar epiphany. The only definition of the word "man" I care about is:

adult, male human

Now when I ask "am I a man?", the answer is really simple: yes, I am an adult male human. All my flaws and virtues are uniquely mine, not some broad category of unmanliness. I have to give my personal growth the individual attention required.

If someone accuses me of not being "a man" or "manly", I laugh inside, because that's obviously false by the only definition of "man" I care about. Their words get translated in my mind automatically, to:

That man isn't doing what I want him to.

Defining "man" as something more, some intangible essence, turns it into a tool for manipulation.