Here’s my theory behind it: Roger and Garp were essentially two massive gigachads in their days. Middle aged men who duked it out without devil fruits. Garp throws hands and Roger swings his Supreme Grade sword.
Roger’s been confirmed a user of all three types of Haki, and we’ve seen him use contactless blows using Advanced Conqueror’s Haki coating. This either means Garp had such overwhelmingly powerful Armament Haki that he could match a Supreme Grade sword coated with Conqueror’s Haki with just his fists, or, and far more likely, he also possesses Conqueror’s Haki and literally matched Roger, blow for blow.
Here’s my other sort of supporting evidence.
Bloodlines seem to play a role, although it doesn’t appear to be completely necessary. Rayleigh said that you must be born with it, which only a select few are. If it’s completely random, the chances of Conqueror’s Haki being in the same bloodline are insanely minuscule. Yet, somehow the odds line up semi frequently. Big Mom has it and so does Katakuri, one of her strongest kids. Kaido and Yamato too. Roger and Ace as well. If it’s one in millions, how come three notorious pirates have it, as well their kids.
Luffy has it, and while we don’t know if Dragon has it or not, or whether he’s the actual son or son-in-law, it might mean Garp has it too.
Last bit of small supporting evidence. Garp’s been offered the position of Admiral several times, which he’s turned down due to disdain of the control he’d be subjected to. I doubt he would’ve been offered the position if he didn’t wield it.
The Admirals are one of the balancing forces of the world, in tandem with the Warlords (now Seraphim), against the Emperors. There’s no way these beasts don’t have Conqueror’s Haki, otherwise they’re at a disadvantage, even with their overpowered devil fruits, because their enemies have overpowered devil fruits too. Sengoku’s been confirmed to have it, and he was an Admiral for a while.
So yeah, I think we’ll get a few panels of Garp wreaking havoc on Blackbeard’s island using contactless punches and outward bursts for the fodder.