Even if that was the reason they made this test (which we have no reason to think it is), that does not discredit mouz at all. A drug test is a verification, not an incrimination, and when it's part of a standard procedure (which it is, drug testing has been on the ESL circuit since 2015), it's not even a suspicion, it's simply routine.
It would discredit them if the test turned out positive, and it would be mouz's own fault for taking an unfair advantage over their opponents.
The private opinion of ESL regarding mouz's abilities is irrelevant, that's not what's going to affect mouz's credibility.
But even if it was relevant, we don't have anything to even guess their private opinion of mouz's abilities, assuming that the test was done because they don't think mouz were capable of doing what they've done is just that: an assumption.
You would need far more than just "mouz got tested after a good performance" to justify accusing ESL of underestimating mouz or even trying to discredit them. It doesn't even make sense from a business standpoint, you don't try to discredit people attending your own tournament out of the blue, that's not good for your brand.
An example of trying to discredit mouz would be, for example, a tweet by ESL's account after mouz's win saying something like: "Wow, that was weird, we'll investigate mouz right away for unfair advantage! #onthegear" or something like that. That would be discrediting mouz's reputation because it would be an authority in the matter directly and publicly disclosing their suspicions about mouz. As far as I know there was nothing of the sort, all the info we have is that they've been tested, as it's part of ESL's procedure.