The main jet does not start to flow "almost instantly". The slide has to be pretty far open for that to happen, and the diaphragm won't raise it until there is sufficient flow. Initially the pilot circuit is supplying fuel. As the throttle valve is opened further the needle jet circuit is active. Only after that does the main jet start to be the controlling fuel supplier.
Reread what JKV45 said about adjusting the idle mixture, you missed some of his points.
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