The reason for bitmixer.io's closure has been outlined by themselves here:
https://bitcointalk.org/index.php?topic=2042470As far as I'm aware, Bitcoin tumblers are legal and no such service has been seized (or at least the public is not aware of it). I could be wrong, but logic dictates that that guy is.
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.