Safari's JS engine is different than Chrome's though, no?

Yes, although I'm not sure (and can't find god info on) whether they started from a similar lineage, or even if they just happen to just share some similarities that Mozilla's doesn't. If SquirrelFish (which became Nitro) happened to start with a similar architectural base, that could predispose them to make similar choices over time (e.g. if arrays are extremely efficient because the underlying types are well implemented, that could have survived and influenced later decisions. Array performance regressions might be seen as bugs that stop progress, and would possible cut of possible avenues of advancement).

There's a decade of history in these engines with a lot of man hours put into them, and they were actively competing with each other very loudly for a while. There's little you could tell me I couldn't find some possible narrative justification for if I consider the PR, bureaucracy, talent and design decisions that might stem from those over a decade.

V8 and JavaScriptCore were written entirely independently; there was little influence between the two. V8 started as an empty codebase, not a fork of JSC.