"He is sitting" describes that "He is in a sitting position"?
And if we want to describe the situation that he is in the process of taking a seat, do we have to say, "He is taking a seat"?
Is this correct understanding?
Your logic is good, but it does not necessarily hold, particularly when 'down' or another adverb of movement is included.

He was sitting down when I pulled the chair out from under him. -- Here, he may have been already seated, but more likely he was taking a seat.
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Thanks Mister Micawber!!

I see..

I forgot some important factor, "another adverb of movement included"...
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