That's pretty obvious.  They are offering a mixing service, which is unrelated to those online services.  If you're talking about using those services as mixers, then it's obviously because they don't want you using them as mixers and could suspend your account, as well as the fact that Chipmixer lets you have the private keys and fiddle with the amounts to anonymise them better.

Also there's the fact that in the unlikely scenario that Chipmixer did turn out to be a scam, it would be extremely hard to tell whether or not all the accusations were accurate or just a lie.  Bitmixer makes it easier to prove with the letter of guarantee, so this is quite a flaw.