I've played this game called Heroes of the Storm for many thousands of hours and I still struggle to identify a player's skill level. The only time I'm certain that I'm playing against a Grand Master-tier opponent is when the match-up allows for a lot of counter-play, these players manage to make you feel incredibly frustrated at every opportunity. It's also one of the best ways to learn, because it teaches you what kind of behavior is allowed and possible, and how to punish your opponent for their mistakes.
In games with elo ratings the skill level is quite clear. If I play vs a GrandMaster then I will lose 99% of the time. I can watch the replay and see what he does better, but even then I will probably understand just a part.
It's not as clear in a team based game, since draft and teamwork both play a large role in the match outcome. Many lower ranked players are often very skilled at micro but fail to macro correctly.
There are streamers that do seasonal Bronze to GM runs, and sometimes viewers will accuse someone of being a smurf because of how well they're playing. But then after the match their profile gets checked and it turns out they're actually playing in their correct rank, and they just got a very lucky draft and overall game.
Finally, even if you're playing vs a GM they might not be playing their main role / heroes, or going the most optimally tryhard builds. Even if someone has the core fundamentals correct, mastering a hero and learning all of their interactions perfectly could take dozens to hundreds of games.
An example from another game: in Brood War there was a time where the most legendary player, Flash, held two of the top 10 spots in the ranked ladder; once for Terran (his main race) and once for Protoss (his second best race). But while his Zerg is also very good, he's not a top 10 in the world player of that race.
