In a word, yes. It ranks among the ATG "David vs. Goliath"/"smaller man upsets bigger man" victories throughout boxing history, alongside the likes of Armstrong-Ross & DeLaHoya-Pacquiao. Choynski may have been past his peak (just as Ross & DLH were) but he wasn't yet considered washed-up & still had enough left to KO an up-n-coming Jack Johnson after that. Judging from contemporary reports, Walcott's win sent a shockwave through the boxing world & probably did the most of any single win to establish him as an ATG/HoFer.
Probably not, rumour has it that Choynski was badly injured\ill going into the fight and from what I remember when researching this fight, that fact was quite rigorously documented. I hope I'm wrong, since it is a sensational win for the Barbados Demon if Choynski was peak.
1900-03-04 The Cincinnati Enquirer (Cincinnati, OH) (page 29) On the other hand, it is shown that he voluntarily boxed for the Dixon benefit with Muldoon's Cyclone, and that he made a good showing with him, something which he would be wholly unable to do were one of his ribs broken. The night of the contest I heard that one of his ribs was sore, as I also heard that Walcott was not in first-class shape, he suffering from an attack of hemorrhoids. Shortly before the men entered the ring Tom O'Rourke came to the press stand in which I was seated and told the reporters that Choynski had refused to box unless he was guaranteed 75 per cent of the fighters' end of the gate receipts, win, tie or lose. * * When Choynski entered the ring I sent a messenger to him to ask him how severe was the injury to his ribs. One of his seconds or handlers climbed down from the ring into one of the boxes and told me that "It's a bit sore, but nothing to hurt. He's all right." Joe has known me for 10 years, and he knows perfectly well that I am not afraid to state any fact about a boxing contest without fear or favor, and if his ribs was broken then was his chance to make it known to the world and get credit for fighting thus crippled, but as he said nothing of the kind I am not inclined to believe that his rib was broken at that time. I am rather of the opinion that the injury was inflicted by Walcott during the contest. Macon.
There is always some kind of excuse after a fighter loses. On the matter of the rib injury, we need to keep an open mind both ways. Walcott did publicly offer Choynski a rematch, and it clearly didn't happen.
In fairness, the beating Choynski took appeared to make him into a shot fighter. It doesn’t seem like he ever made it back to his relevancy before 1900.