It's a very simple reminder. No deeper meaning behind it. I just want people to come to terms with the trauma. There are some unpleasant truths in DB. This is just one of them (another being Gohan is the strongest unfused fighter in canon
). Sort of like "SSJ" being wrong in every language (officially, too).
Some take it to imply Vegeta cannot lose to Goku, Vegeta is stronger than Goku and a variety of other things going beyond the simple statement of fact (not wanting to face the truth). While all that is fun, I'm just offering counseling here (a chance to face the music). It really shouldn't be such an effective way to get a rise out of so many, yet it is (time and again).
Hiding behind other characters, deflecting to other areas of interest, scrub talk, favoring the subjective over the objective and so on. Anything to avoid accepting the whole of AT's will. Frankly, I don't think the old man even knows he did this.
Were he asked, he would think Goku really won against Vegeta (both times, even). He certainly wrote Vegeta's EoZ speech to suggest a bunch of things were so (that just weren't) as with Goku's motivation for having a former villain spared (just wanted a rematch). Still, I'm not complaining. I don't mind this eternal duty.