This is the same exchange that a few months back blatantly encouraged wash trading with its "airdrop" of its own coin, this looks basically like the same thing, wash trade $20k (So that's what, 0.5 BTC back and forth 10 times?) and get "free money". Just a marketing ploy to create false volume, they'll earn back a bit from commissions and of course from unsuspecting new users who see Bithumb high up on the charts.
I think this sums it up perfectly.
You're essentially being incentivised to trade on an exchange which has allegations of fake volume just recently, as well as potential manipulation within the exchange.
Not only do you not have any point in doing so given that you are most likely going to be paying higher commissions than what you are receiving, you are also probably handing over your identification documents to an entity overseas which you don't necessarily have any knowledge of. Also, there is always a risk that they may freeze your funds while they are deposited, and since they are operating from another country, there is pretty much no legal recourse that can be taken.
Given all of these factors above combined, I would not take up their offer, even though on the surface it may look like it's "free money".