I have had limited discussions with @markymark, and I have to agree with your assessment of his flagging of you. I suspect - I do not actually know - that he may feel you have insulted him personally via ad hominems, rather than by simply your opinions. I tend to refrain from ad hominems, and despite the extremity of my criticism of his botting he has refrained from flagging me, at least from any account I am aware he runs.
He seems to be fully taking advantage of the free flags, and while it is of little value to flag the kind of user you mention specifically, it does have some value and does not harm the platform. Flagging you does harm the platform, and doesn't provide any benefits. It's an example of how the flag mechanism is broken.
Thanks!