In all honesty, any respectable broker is highly unlikely to be able to fill any position at the price point desired during a flash crash, it’s just impossible. The liquidity is thin with massive jumps between liquidity gaps. This doesn’t always mean that a broker is corrupt per se, it’s just a mechanic of the markets - unfortunately.
Moving on from the above, which is in this case gospel, it would not cross my mind that some “less than worthy” brokers will take advantage of this. However, trying to identify foul play during such a crisis is near to impossible using the information that retail traders have to hand.
It’s a tough old game, but we have to take our edge when we have it - during a flash crash is certainly not one of those moments if your on the wrong side.