mine was $200,000 on Stake.com 2 weeks ago
Was this a flex?

Yes this is a big flex mate and I don't believe that that member truly lost such an exorbitant amount on stake just 2 weeks ago, this should probably be a make-up story and figure because that figure is too high, $200,000 in a go as a lost is such a huge amount for anyone but I know is not impossible but in this case I can see a clear impossibility in the event unless if the member decide to share some more evidence to support this claims.
To be honest, I also don't really believe it if there is no really accurate evidence that OP says $200,000 is a really big amount of money in my local money, spending $200,000 on gambling if you are not a really rich person it will definitely make you was frustrated that
I even losing around $300 made me regret it for the rest of my life and I will never repeat such excessive gambling activities again. Maybe this amount doesn't mean much to someone who can afford it, but for me it is something that is very suffocating.
as you said, there is a lot of impossibility when someone spends so much money in just 1 session, is he really risking such a large amount so that it just runs out, even people who are addicted to gambling don't seem to spend their money in 1 session (IMO).