If you're telling me that anyone can be as good as someone else who is equally smart but with 2-3 years of experience with that specific tech stack, I'd say that can't be true, almost by definition.

It's a new language. Syntax, semantics, common patterns, high level approaches to things (you could call this culture).

It's a new VM (possibly) or compiler.

It's a myriad of libraries.

It's operational tools around the main tech stack (debugger, tracing/performance tools, deployment tools, logging tools/frameworks, etc).

To truly master a mature software stack takes years. Yes, a smart and experienced dev can hack their way around with the help of an experienced team, but the real test will be when the same dev looks at their own code from the start.

Though even that's not a sure thing, because if it's one thing developers have plenty of, it's hubris. Sometimes it helps, but many times it doesn't.

> I also don't think the claim was that our candidate would perform at the level of a senior dev.

No, the claim was even funnier, that the candidate will be a competent developer because he has a PhD in physics (a field somewhat related to software engineering, if you squint really hard, but not really).

On one side you pit a "real senior programmer /architect with years of active product design and development under the belt" and on the other side "person who learned to screw size 8 bolt into size 8 nut and been doing this and nothing else for years".

My point is that for this situation:

* person A (truly senior polyglot developer just now starting to use a specific stack)

* person B (truly senior developer with experience primarily in that specific stack, all other things being equal)

Person B will smoke person A at the start of the project, since they're already ramped up. Person A will have to learn, that's why we call it "ramping up".