Someone just bought the account, can't really do anything about it. It will probably take sometime to get him to his real MMR since that account hasn't played much comp yet comapared to the spike rush matches.
Smurf. If you look at his spike rush matches you can easily conclude his MMR is very low so when all of a sudden he starts playing insane, his actual MMR takes time to adjust.
Basically despite the rank, he is playing against low MMR players aka shitty players that's why the consistency
He played against a 39 tracker score Reyna.... But eh not my problem, if he is indeed cheating then he will probably get banned whenever the next ban wave happens because I believe the anticheat is good or you could tag the vanguard guy on X I think he is pretty active there