Going from 20k to 10k in 2 months doesn’t seem weird to me at all if someone is playing a lot of games and studying some too.
At my “local” go club, there was a guy who had very recently learned playing at 20k in March, and by mid summer was at 10k.
I’ve seen a number of other people reach 8-10k levels from 20 or 25k within a few months. It’s about how long I took back in the day (though my 8k AGA in 1990 is probably more like 12-13k today on OGS)
In any case, out of a large pool of beginners, I would expect at least a few of them to reach ~10k within a few months. That’s a pretty typical progression speed for a promising beginner who takes to the game quickly, and is likely to eventually reach dan level.
It’s also not at all surprising that one such player might win all their games! They were probably at the top of the beginner range to begin with, and very quickly improving already. If they weren’t paired with another such player, they would probably win.
This just isn’t suspicious at all by itself.
Suspicious would be if they were playing and beating ~10k players on even almost immediately after joining the tournament. Or maybe if they were at dan level after a few months.