First of all, the phrases "as it can be said", "as it is a fact", and "being a true fact" are wordy pompous monstrosity that would never be correct on the GMAT SC in a million years. They are precisely the sort of indirect bloated overwordiness that the GMAT hates. Those three phrases should be taken out back and shot. (A) & (C) & (E) are out right away.

Now, let's compare (B) & (D) without the adverb (coincidentally/incidentally) and in the flow of the sentence:
in (B) ..... who is to be well known for his ....
in (D) ..... who is quite well known for his ....

The first is a very unusual phrasing. All we want to do is state a plain ordinary fact, a fact about how Ali's fighting style is known. The "is to be" construction, first of all, is exceedingly uncommon on the GMAT, and moreover, it is used only for strong recommendations, often rooted in political/ ethical/ religious convictions ----
"A woman's rights to safety in the workplace are to be upheld under all conditions."
"The American flag is to be venerated."
"People joined by the sacrament of marriage are not even to think adulterous thoughts."
Here, in this SC sentence, we are not making any strong moral statement about how Ali "should be" thought of --- we just want to state the simple fact: this is how folks do think of him. Totally factual.
The only logical choice for a plain factual statement is answer choice (D), the OA. BTW, the words "coincidentally" and "incidentally" share the same etymological root and, in this context, mean more or less the same thing. I think the sentence would be even better without these words, but the point of GMAT SC is not to imagine the best possible sentence, but rather, merely to find the best choice from among the five given.

Does all this make sense?
Mike
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Mike McGarry
Magoosh Test Prep