This guy is the prime example of a jelous, braindamaged and delusional 3rd rate fuck.
He does give an explanation as to how this would have all been possible.
Favoritism: he pretends that the most powerful man in the country decided to gift some unknown treasure to a random weakling that allows said weakling to break all records and accumulate unprecedented achievements, all in solo nightmare instances.
This is obviously baseless, and even contrary to known rules in-universe (there are level requirement for items, so there can't be such a powerful equipment that would still be accessible to a level 17), but his inflated ego must find excuses to deflect from his own shortcomings. If there were such a stupidly powerful item without level restriction, it would be so valuable there would be no way it would be granted to a rando without remarkable background.
For reference, the most powerful
known item accessible to him at this stage was the ring that could be crafted from the Goblin King's stone. Which would have been good (though he didn't even get it in the end), but not enough to allow a single weakling to substitute for and overwhelmingly surpass a whole team of elites. Imagine what an amazing item would be required for this to happen. And the kind of person you would give it to.
And because he asserts this all without a single piece of evidence, his jealousy is blatantly on display.