This seems accurate to me. A senior dev can move between tech stacks and not loose their title. At the same time, to me that same senior dev will be the one that recognizes when they are still learning a tech stack and won't be afraid to ask questions of anyone that might know, regardless of that person's title.
I think the difference is you can be a senior developer, but be learning a new stack or learning a particular technology. The key difference is you will recognize the patterns and practices that generally make code more stable, maintainable etc. A person with 1-2 years of total experience will make more mistakes learning any technology than a person with 10 years of experience.
Yes they do. Generally they're not considered senior because they have X years specifically dedicated to Node.js, but instead they have X years of proven development experience with the last 2+ focused on Node.js.
It depends on the amount of things and deepness the developer has actually experienced/seen/solved/set up with that particular framework.
I wouldn't assume someone is a senior only based on the years of experience. I've interviewed candidates with 5+ years PHP who are still lacking basic knowledge to be a senior.
Start with: what does senior mean for me and my company?
You can be a senior, very experienced developer using a particular tech stack without all of that experience being developed with that tech stack.
In fact, I'd say at a senior level a developer should have considerable breadth of experience and the ability to comparatively evaluate tech stacks, not just skill in using a particular tech stack.
It's not like being a senior dev in language or framework A magically means you're a complete and hopeless noob the second you switch.
Sure, you'll have rough patches, but being a good senior dev specifically implies that you know yourself well enough to handle such transitions.