If you can simply patch it, that's mutable, not immutable, by definition. Immutable means unchangeable without exception.

You are misunderstanding what immutable infrastructure is. You don't buy some SSD with a preinstalled OS and use it as it is for 10 years.

The point is in not making changes on the fly on live systems. You are perfectly able (and encouraged) to deploy new nodes with updated software and roll over the old nodes.