he is smurf he probably got gold 1 after promos anyway started to play few days before the season started on that account and won many games were he ranked up to master at least before the season started and got good MMR before the season started so he plays with high MMR ... usually, you will see account like that being sold and usually, this kind of players spam only 2 -3 champions to get that rank if he had no match history in few days probably selling account since its new one and has that winrate for grandmaster, you can search selling accounts and you will see most master and grandmaster accounts have 60 -70%winrate and sell it for a lot of money ...
the second theory is that is a good challenger player that is casually smurfing in the start of the season... if you tell me that is not possible to get out of gold 1 to grandmaster in less than 3 weeks you are trolling hard... you can open almost any semi or pro player making new account and smurfing from iron (new account to challenger in 4-5 weeks )
If that account has a small number of games in last season and huge winrate that its smurf and my theory is true (one of the 2 cases ) players that are high elo abuse the last few days of ranked to make a new account and play until they do promos (while season end ) in preseason they play more and get higher rank and mmr that is carried after its soft reset (new season) probably that guy had already master or grandmaster rank in preseason