I agree that Docker actually has a relatively good track record, but it is true that Docker will never be on the same level as a VM hypervisor that was designed from the ground up to be a security barrier.

Thus, when people bring up "Docker is insecure" I try not to get down in the weeds arguing about the specifics, and instead point out alternative projects that are designed with security in mind. I find it's a much stronger counterargument.