Amir's young age has to be considered here. Nobody at 17 and 18 should be expected to have world beating numbers. One could clearly see him get better and he had already shown that he was capable of making an impact.
And I agree, with whatever we saw of Shane Bond, one could make a good case about him potentially having been one of the greatest ever and that's fine! Does it mean he was? No, but he gives reason to believe as such.
Similarly with Amir, we are not doing ifs and buts, it's making an assumption based on what we saw of him at such an early age. Despite not having amazing numbers overall, he showed what he was capable of.
My point was, we should compare Bumrah with someone whose career wasn't curtailed like Amir's. Sure, Bumrah lost a couple of years to injury, but he had already established himself and set his foot in his career. As a teenager coming up, your early 20s are instrumental for growth and development and that's when players begin making something of their career.
I still think that this is an unfair comparison, and I'm not even saying that Bumrah wouldn't be the better bowler anyway.