Yeah, you can track the debuff in FFLogs. It's called Luminous Aetheroplasm. It'll even show you a chart of the max amount of stacks they reach. Here's one with 4 for example:
Dave Clark Explodes.
I distinctly remember the 1586 log had no one above 3 nor were there any deaths attributed to Exhaust, so I ruled that exploit out. As the log was recently hidden again, there's really no way to further scrutinize it and we can only wonder now. But from what I remember, the guy was able to cast ~20 more Fire IVs (around 115 total) than the #2 BLM with a longer kill time (12:16 vs 13:04), had 22% uptime on Foes and his Enochian uptime was at 90-91%.
The first fact alone guarantees around 100-150+ DPS over his peers, but the frequency at which he was able to cast it vs. others was really strange based on what the posters above have said. Judging by his old lodestone stats, he had around 790 Spellspeed with 3 Diadem pieces, but I've seen people with way higher and they still casted less Fires than that guy. Either he's hiding multiple god-tier Diadem pieces and is the real deal (which I imagine can be likely) or there is something we're unable to see at play. It's just unfortunate he chose to hide his log.
To be fair to the guy though, he did manage to post a
1475 A3S which is the highest without an AST. So I imagine if he's capable of this, having a top parse in A4S shouldn't be that far fetched.