Service meshes make in-cluster mTLS a more-or-less automatic feature, which is worth having. Some will terminate TLS at ingress and convert to mTLS internally. The argument above is that you shouldn't do this and should instead plumb that ingressing TLS traffic all the way to the container.

The downside of plumbing directly to the container is that you lose many of the routing features of a service mesh. If it can't inspect the traffic, it can't do layer 7 routing. It can only route and shape at layer 4.