The Linux kernel has far too large of an attack surface to be trusted as a hard security boundary. It is good enough to prevent mostly trusted software from accidentally interfering with each other but I would not trust it to protect me from an untrusted workload.

For example GCP and AWS both have container running services. They both use hardware VMs to isolate different tenants. You will never share a kernel with another customer (I don't even think you will share one with yourself by default).