This would make sense if we're talking about a non-custodial wallet like Bisq, which uses multi-sig to "escrow" fund for trades.
Note that Bisq doesn't have this model anymore, which was a 2-of-3 multi-sig, in which the arbitrator held the third key, because this gave way too much power to the third party. The arbitrator could pretend to be a simple user and steal the funds without question.
Instead, since
v1.2.0, it uses 2-of-2 multi-sig, wherein if the trader and the maker don't find consensus, and neither does the mediator help, the money are sent to the arbitrator after a specific period, which is the worst case scenario.
Isn't Wasabi supposed to be a completely non-custodial wallet?
It is completely non-custodial wallet, in the sense that you have access to the private keys and that it's open-source.
In technical terms how is the wallet able to prevent someone from sending tainted funds to the join pool?
The wallet doesn't prevent anything; it's the coordinator server that does.
Does the pool just send those funds back to the address from whence it came?
The coordinator probably just refuses to include you.