According to the Portland Police department, they receive an anonymous email claiming to be mixing the shakes with cement and "other harmful chemicals." They put out a tweet asking for anyone hit by one to come forward.
As of this post no one has come forward.
So. claim made via email. Police put out a tweet asking for evidence. No evidence has been presented.
Conservatives be like "ooh no, it's real! It's all real! We were just hanging out with Proud Boys, being racists and all, when someone threw a milk shake at us! I mean, no, that one didn't have cement in it. But the cops said that someone said that they put cement in it so it must be real!!!! IT MUST BE REAL!!!"