Sidious is one of the more explore characters in Star Wars. Also, only a few star wars characters are viewed by many as being able to stand up to Sidious. So the question I have is:
Has Sidious ever defeated a force user who was near, equal, or greater than him in ability and power?
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
I didn't see a single white helmet in the Rotunda until after Yoda fled when last I watched ROTS. And no source I'm aware of attributes his sudden departure to the arrival, impending or otherwise, of mooks.
Yoda clearly didn't have all day to get the job done, but Imperial security was tragically lax, since the duel went on for some minutes without intercession from troopers. And Mas Amedda didn't seem to be in a hurry to run for help when he left the holding office beneath the Rotunda.
Personally, I'd say it owes to a number of relevant factors: Yoda was confirmed to be unarmed, Sidious might not have been, he'd just fallen from a great height (consider the amount of time and effort it took for Yoda to get to Sidious during the earlier pod scene, Sidious would just likely return to his missile-whoring ways), suffered a severe blow to morale (ROTS Visual Dictionary), and had an epiphany that a protracted confrontation with Sidious wouldn't get the job done (ROTS novel). And then, yes, that stormtroopers wouldn't be twiddling their thumbs all day to lend the boss a hand.
Basically, a bunch of shit fell on Yoda all at once and he made the safe call.
Yoda had little energy left after the fight to continue (thanks to his age and use of Ataru), otherwise he should have had the strength to pull himself up (Sidious was also hanging off). Granted, Yoda had a shot at winning if they had been on even ground like Mace was against Sidious.
And that translates into a Sidious win? I guess this IS semantics. While everything you said may be correct in that Yoda didn't have the time or the patience to get back all the way up, it doesn't appear Sidious wanted any part of Yoda either after his O face.
Sidious accomplished his primary goal whereas Yoda, by his own admission, failed. Sure, Yoda didn't get his ass outright spanked (no one suggests that). Likewise, Sidious's O face means no more to me than Yoda's look of extreme pain that preceded it.
Lets not get into semantics here. The fight itself was a stalemate, regardless of greater implications. And while Yoda did have extreme pain, it's a little different when he turned that around onto Sidious.