Shaun's a very good guy and not someone who likes to make up excuses... I've heard him say many times "I just played like a donkey" after losing in the local tour stops. Hell, I'll even suggest he caught a bad roll or something and he'll say "nah I just shot like shit."
I think at one point he had his own issues and overcame them, and is proud to be living clean for many years now. It's a subject that upsets him more than most. But it's not like that's all he talks about, he's had a lot to say about rigging the rack (and that was a legit complaint too). And he also talks about players behaving professionally, dressing better, etc.
Maybe he had a specific match in the Open that prompted that post, I dunno. We've seen those occasions where someone is getting run over, calls a timeout, and comes back a new man. There's really no way of knowing whether it was just a pep talk or a pill.
The idea of testing seems pretty impractical. If you test before the match, they can take something in the middle during a restroom break. Even if you test every few hours, there's probably nothing that can spot every possible useful drug immediately and inexpensively, and certainly nobody's gonna want to do a blood draw in the middle of a match.
So he's safe enough woofing to play anyone in a drug-tests-every-2-hours match since nobody's gonna play with such a weird stipulation.
I dunno what separates him from the top tier, on some days he looks impossible to beat. But for years he's been stuck in that middle tier. I don't think the difference between him and the top tier is chemical, maybe it's just mental.