I agree, there is room to speculate due to Rocky's character. The possibilities of him having lost legitimately, having let Apollo win or having won but then lying to Adonis, they're all valid. But the movie itself presents it rather bluntly and isn't all that concerned with it; otherwise, it would come up again as a plot point, perhaps related to Adonis's confidence or something.
As for Rocky vs. Apollo: in the first movie Apollo is arrogant and hasn't trained. He's also older. Advantage Rocky. In the second movie Rocky's advantage is stamina and willpower, something that always benefits him. Apollo is determined to take Rocky out quick, so when he doesn't it's a war of attrition at that point. Mickey also teaches him footwork and how to jab with both hands.
Where I really wonder about their second rematch is Apollo in the third movie has taken Rocky, who up until that point isn't actually very skilled, and given him speed and agility. You know, taught him how to "fight black". So if Rocky, who always had the strength and stamina, now has the skills it would be an impressive fight. The only thing is I can't imagine it being much more than a sparring match, like neither man really tries to hurt the other (we see Rocky at the beginning of IV has some bruises, that's it), and there's the wildcard of Apollo claiming he didn't teach Rocky everything he knows.
As for Rocky IV, we don't really see Apollo training. It could be another instance of arrogance, thinking he can take on this rookie Russian no problem.
Also, it depends on how Stallone is feeling when he writes the script.
Which is what makes Creed fascinating, in that it's the only one that isn't written by Stallone but does stay very respectful. But what I said before is it always seemed like Apollo was a highly revered boxer, but in Creed it positions him as basically the greatest of all time. But Rocky Balboa had already done the same with Rocky a decade earlier. So which is it in-universe?
It's hard to say, because Apollo is supposed to be a little older circa 1976, so he would have been a contemporary of Muhammad Ali and Joe Frazier, and would have fought a younger George Foreman presumably. So when Joe Frazier cameos in the first Rocky, are we supposed to think the gentle ribbing between he and Apollo is because Apollo is an Ali stand-in? Or is Apollo the one that finally took Ali's title?
I do like that this series has somehow managed to stay tightly consistent over the years. All except for Robert's fluctuating age in those mid-movies, hah.