Yes, it does sound like a joke, but so is Oda, so everything's possible

Sure, it´s fiction so in principle anything is possible. But have people really considered what this would entail? Garp pretty much is overshadowing everyone in the flashback except Rocks/Roger, and Garp is probably above pretty much any midgen top tier, including his own super hyped up son, and probably even the current MC
For Koby to surpass prime Garp, he would esentially need to be number 3 of his era, and would have to start out preforming Zoro, Law,Kidd Sanji, and even harder maybe, Loki, who could end up being as important as Luffy and Teach in what he got simply at birth. Keep in mind, this is a character we don´t even have any reason to see above Doffy/Cracker level fighters, who were last relevant a decade ago due too powercreep.
And the other issue is that Oda isn´t really setting up the whole Old gen losing thing. For example, to do a new generation should surpass the old situation a ¨Goku¨ like character would need to lose to a ¨cell¨ like character for the new gen character to beat. But Oda hates having characters lose 1v1´s fairly, as you can see when he revealed Roger never got fairly caught, Oden got hostage diffed, Gaban got hostage diffed, Garp got ganged up on, Rocks lost unfairly, Rayleigh has just never lost and humiliated modern day villains, lol.