That guy is active mainly on illegal markers subreddits, so it could be LSD talking.
I know that mixers are legal, but while they are not illegal yet, it's just a matter of time before they become illegal and in that case BitMixer.io would be the first target. That's probably the reason why the admin decided to stop it while it's safe.
The mixing sites are used mainly by people earning their coins from illegal deals, so i think with time governments will illegalize such serv ices, even if it is legalized till now in many countries.
There's no reason for mixing sites to become illegal. While probably 99% of people use them to buy things from the dark web, the services themselves do not endorse/represent any such thing, they just provide privacy.
Bitmixer.io most likely closed down because they were aware of how people were using their service, at least that's what I can infer from their post.