I'm not exactly sure what you don't believe, your comment is hard to follow, or relies on premises I haven't detected. What you are describing in your first paragraph is somewhat reminiscent of dynamic dispatch, which Julia does use, but generally hampers performance. It is something to avoid in most cases.
Anyway, performance in Julia relies heavily on statically inferring types and aggressive type specialization at compile time. Triggering the compiler later, during actual runtime, can happen, but is certainly not beneficial for performance, and it's quite unusual to claim that it's central to the performance model of Julia.
If you are asking why Julia allows recompiling code and has dynamic types, it's not for performance, but to allow an interactive workflow and user friendly dynamism. It is the central tradeoff in Julia to enable this while retaining performance. If performance was the only concern, the language would be very different.
Anyway, performance in Julia relies heavily on statically inferring types and aggressive type specialization at compile time. Triggering the compiler later, during actual runtime, can happen, but is certainly not beneficial for performance, and it's quite unusual to claim that it's central to the performance model of Julia.
If you are asking why Julia allows recompiling code and has dynamic types, it's not for performance, but to allow an interactive workflow and user friendly dynamism. It is the central tradeoff in Julia to enable this while retaining performance. If performance was the only concern, the language would be very different.