"Petrosian has an equal record against both , and this after he was demolished by Fischer"
Fischer was -2 against Petrosian until his late 20s in the 1970s while Kasparov was -2 for a part of his teens (after blundering away two wins) and then won their last games, from his later teens. In 1982 Petrosian was less than 100 Elo from the #1 spot while he was 140 points from it ten years earlier, so Kasparov didn't play too badly against him, getting winning positions in all four games they played 1981-83 even if he only won the last two in the end.
Magnus could bring down the whole empire of monsters. I dont think Giri is old enough yet. Magnus is stronger than only his own creativity. He have read loads of classic and modern games and remembers everything, on top of that he is a supercalculating genious.
If Capablanca had all those games in his head, he would have been a hard task for Magnus. But he hadnt. Those games wasnt played yet.
Magnus is something like Capablanca, with tons of GM games added in his visualisationbank. Capablanca had probably less than 10 000 GM-games in his memory.