I understand how ending up with bitcoin from an address related to a an address linked back to illegal or immoral activity could make you seem suspicious of something and how the receipt can claim the money is sourced from a mixer. However, couldn't this also be used the other way around to say that the money was gotten from a mixer and therefore it was originally from an illegal activity/money laundering.
Using mixer is like using encryption - it is for privacy and it is not illegal (yet). Same argument could be used with Bitcoin - you have Bitcoins so you do something illegal/money laundering. It is drug dealers and terrorists currency! Governments wants to ban encryption and use the same arguments. Proof that you used mixer is the same as proof you have encrypted hard drive or you use https.
it comes down to a judge trusting an anonymous source (BitMixer or ChipMixer).
As long as judge understands cryptography it comes to judge trusting cryptographic proof of a known identity. We are a witness. We do not have an interest in the case. We have no reason to lie. Our public key is known and unchanged since the begin of ChipMixer. If users trust us with their money based on our reputation then why judge would not trust us.
Playing devil's advocate you could argue BitMixer can still use their address to sign from after closing, and that signature can say anything they want.
Yes, they still can sign and it should still be trusted. Satoshi also went "out of business" but if somebody signs something with their key then we would trust that.