Sorry, but that makes even less sense. You said yourself he must be using the diuretic as masking agent. So he's not using it to make the cut easier: he's using it to prevent USAD to find another PED. I don't say that's the case, but your argument is irrelevant if the diuretic is a masking agent.
In my head, a masking PED is not more excusable: it is used to be able to cheat without getting caught. That's why the penalty are not softer: we can't tell if the fighter was using other PEDs, they are masked by the diuretic.
We should not excuse fighters who test positive for it. It's not a minor infraction if it's used to hide steroid use. Get it?