How common is boosting?

Was playing with this diamond 1 malthael the other day and he was legitimately one of the worst players I've seen in recent memory. He struggled badly to keep up with TLV on soak, constantly tried to kill the TLV (and failed) and got ganked multiple times in lane due to overextension (bad vision control and awareness). Ended up having tons of deaths and almost no impact.

This got me thinking - how did this person ever reach this rank to begin with? I mean technically he's more or less a "master". There's just no way he did it solo queuing. I'd be surprised if he could even get past modern day smurf silver solo queue - I'd place his true skill level at around Silver 5 or 4.

I see two possibilities:

• Got boosted by a legitimate super-GM (I'm talking like top 5 world-class player skill). Even then, I feel like it would be quite challenging to carry this guy all the way up to master. You'd be playing at an extremely significant disadvantage against people who would theoretically be similar in skill to yourself.

• Got boosted and reached rank years ago when reaching diamond+ was a cakewalk and since then did just enough to slow down rank decay without playing too much and exposing his true skill and rank. I think this is probably the most likely answer, but then what's the point of even playing HOTS at all? Bragging rights?

Do you guys see any other possibilities?