In the JVM devirtualising means making a virtual object a full object again, so the opposite of what you want to be happening.
I don't think the JVM really names the optimisation you're talking about, but it does do it, through either a global assumption based on the class hierarchy, or an inline cache with a local guard.
When we say devirtualize we mean that we know animal is always a cat so we don't have to look up which speak to use.
And that makes literally no sense in-context, you're mapping those words directly to the concept of virtual methods but that's the opposite of the way chrisseaton uses them (and they're clearly familiar with the concept of virtual calls and it's not what they're using "virtual" for), hence asking them what they mean specifically by this terminology.
Yes... but let's not say 'broken up on the stack' - the object's fields become dataflow edges. The object doesn't exist reifed on the stack - fields may exist on the stack, or in registers, or not at all.