judging from the text i searched, the translation of "the rest is obvious" is quite good, at the same time, "beyond this what is there to discuss" is, appropriate.
in general, the "style" of mr 鄭's text, is mainly literary chinese, with occasional written vernacular chinese "toppings".
then, to decipher "其餘又何論矣"
其餘 - the rest of . . .
又何論 - 需 is omitted / assumed, so it's 又何需論
又 - and / further
何需 - need to
論 - discuss
矣 is a 虛字 ( chinese particles ), to indicate the end of a clause, in an affirmative manner.
so, put it together with the text:
(in 推手), if you're skillful in "彼微動﹒己先動", you're in charge, not your companion (則操縱在我不在彼)
the rest of this (you're skillful in 彼微動﹒己先動), is not needed to discuss further.
ps: i skip some steps in this explanation, either in literary chinese, or in 太極拳.
have fun :)
edited, in lieu of comment.
well, literary chinese is fuzzy. incorporate the context is important, and easier; to explain particular clause.
imo, the clause "其餘又何論矣" is not fine, it's not "pure" literary chinese. the introduction of "需" is "needed", for easier comprehension.
anyway, this interpretation is, based on my knowledge in literary chinese, and, 太極拳. others might understand the phase differently.