i think the truth is somewhere inbetween
yes purge has a very high mmr by global standards, and from the viewpoint of your average dota player yes he would be coined as very good. However, unless explicitly stated so, I think it is fair for players in that same skill bracket to chime in about purge about his flaws and faults, which are quite plentiful for a player of his mmr. Now, remember that solo mmr is not a 1:1 ratio to skill in dota by any measure, and Purge is actually one of these good examples.
Purge is mechanically very flawed. I have seen him mess up skywrath combos, wards, wyvern ulti targets, and a plethora of other things. I am not stating that I have never made similar mistakes, nor that such mistakes is a damning fault of Purge. Instead, the issue is that he makes these mistakes MORE often than a player of his skill placement should make, and many times in a matchmaking game if Purge is your 5.5k he is infact a 'weaker' 5.5k than other players of similar mmr. Akin to how I would have other players of similar mmr compared to players like Rena-chan aka Flame of Despair, or a different 7k compared to iwo. While their rating is good, their performance relative to their mmr is lackluster.
And that's really the whole situation. Purge is 5.5k yes, he is very good compared to the populace, but I personally, along with a majority of other players who actually do get matched up with him, can say he is a weaker 5.5k by a margin.