Yes, the function name says exactly that as well, though admittedly what this actually means and what consequences it might have if the assumption is false are probably fairly opaque to a novice user.
> how does it handle the case where you don't have enough capacity to insert the new (k,v) pair?
Breaking the invariant results in safety-checked undefined behavior, that's what the "asserts" in the doc comment signifies[0]. Basically, if something goes awry at runtime you'll get a crash along with a nice error trace in Debug/ReleaseSafe mode or with the appropriate @setRuntimeSafety call.
If instead you'd like to have errors that you can handle, you could use your real allocator where you expect to actually use it and pass a failing allocator[1] everywhere else (but that's sort of abusing the API IMO, I don't know if I'd actually recommend you do this).
[0] https://ziglang.org/documentation/master/#Doc-Comment-Guidan...
[1] https://github.com/ziglang/zig/blob/0dffab7356685c7643aa6e3c...