Are you suggesting mixers to start hiring government agencies to approve the coins coming in to them then mix the coins of their clients?

Just because a random Blockchain analysis company claims something is gray does not mean it is gray. Blockchain is open for everyone to trace transactions. Why don't they show the transaction reports, the connections, destination addresses everything publicly? They will not do such thing because they may not have concrete conclusion.

How did they even know it ended to address/es that is/are from Sinbad wallet?

Sinbad is the most popular wallet at the moment, it's understandable that they have more clients than other mixers existing right now. So raising a finger to them is easyer than anyone else.

But let's be real, today or tomorrow every mixers will be blamed for something even if they are not guilty.

They don't say an exchange did this/that because exchanges don't care for their clients privacy.