Read again. If what it's written it's true, then their trading engine is exploitable - an exploitable trading engine is NOT acceptable, it's a SCAM
It depends on what you mean by exploitable.
Alice bids 1btc at $10
Bob bids 0.5BTC at $100
Charlie Asks 1btc at $5
In order execution, Alice gets fulfilled, Charlie gets $10 (is that right or does Alice get for $5 or what?)
Out of order, Charlie gets 0.5*100 + 0.5*10 (or does he still get just $5).
I don't really know enough about the matching algorithm to comment further but maybe someone can run with this scenario then explain the exploitation aspect. I always appreciate being educated.