What's not covered here is overlap between what an engineer knows and what the business needs; and especially knowledge of the business's existing codebase and

What's not covered here is overlap between what an engineer knows and what the business needs; and especially knowledge of the business's existing codebase and architecture. If you know what exists and how it's put together (and this can come from tenure or experience in a vertical) you'll be far more productive. Tasks require less research and are implemented leveraging more of the existing design, possibly with a few tweaks in key locations, where an engineer with equivalent or even more talent may rewrite or reinvent a whole module owing to less deep understanding, and actually add less value. This can lead to an engineer deserving to be senior in one company but not so senior in another. It's also a source of technical debt and risk when employees turn over. Knowledge is lost and merely hiring a replacement doesn't restore velocity. Sometimes modules need to be rewritten and replaced to get back to a good place.