What if they dont play seasons, and only play WL? I mostly play during the week to kill time, or adjust to a new player during the week, and get the majority of my coins and players from WL rewards. Its not unrealistic that at this late stage of the game, someone who played WL every week and reached a minimum of gold 2 for 45k, after something like 35 WLs now could have potential coin earnings of 1.5 million before they even opened their reward packs. Most of my TOTS players are untradeable too from the upgrade sbcs, in fact of my team only Herrera (200k) Bruma (45k) and IF Van Aanholt (70k) are tradeable, every other player I got from turning in all of the useless players I'd got in my rewards that I kept in the club just in case there were sbcs that I could use them in (which there was).
You also have people from d1 who relegate themselves to d10 and then promote back up to d1, which is worth something like 150k plus match rewards, so you could just be unlucky and face someone like that