Exactly, a senior frontend web engineer is not going to transfer to senior compiler engineer, or senior embedded engineer, or senior game-dev, without a substantial ramp up period from lack of domain knowledge.
The counterpoint is that if I am proficient in Ruby, and senior level in Node, and work well in Java, I can figure out and be fluent in Python or PHP within a week or so.
Sure if you had someone that was working as an aeronautics engineer designing planes for years they probably would require more ramp up before being able to work on ICBMs, but they would do just fine moving from Lockheed to Boeing.
In my experience I've seen very cocksure senior engineers come in with the mindset that "this is just another tech stack, that I can learn in a week"... failing miserably to recognize that said tech stack is exclusively used in a domain that they're not at all comfortable with.
Arguably XHP/React is successful because some senior compiler engineers plied their trade in the front-end world. Or was it senior front-end engineers digging into compiler tech?
How about Servo pulling a bunch of game development techniques into the browser development world?
There are definitely technical domains you can gain or lack expertise in, but "frontend" vs. "backend" isn't one of them. Those are business contexts, not technical knowledge.