Plasma Transaction Commitment Vulnerability?

I think I can see an issue with the block withholding mitigation / withdrawal mechanism I wanted to discuss.

Source

From the Plasma whitepaper: "...it is impossible to detect on the root blockchain whether a block is being withheld (either the user can assert that they never received the blocks, or the Plasma chain can assert that the user refuses to recognize that the block is available and is lying)... Instead, Plasma presumes that if a user believes that the Plasma blockchain is adversarially withholding blocks and may impact the ability to enforce state transitions in the future, then one simply should exit from this Plasma chain to another as quickly as possible. "

So am I right in thinking that a bad actor can pretend not to have recieved a block, and that this cannot be validated by the mainchain, and by withdrawing they can then rollback all transactions they were involved with on the plasma chain since the last (infrequent) update to the main chain.

If so, imagine the plasma chain is being used for a distributed currency exchange. I could transact hunderds of times on the plasma chain at high speed, then, just before the plasma chain commits, decide that because I lost on those trades, I will pretend I didn't get a block and signal that I want to withdraw. In otherwords, I can engineer a bet to nothing, where I withdraw whenever the market goes against me, and retain the transactions wheever the market goes in my favour.

If so, this is deep problem, as essentially it means that no transaction made on the plasma chain can be trusted until they are committed to the main chain. This would mean that despite appearances the plasma chain in reality only operates at the same speed as the main chain.