You forgot something. We are talking about the compression load on the mast.

With a two to one tackle, one half of the force is exerted on the halyard but the other half of the load is exerted on the masthead. The fixed point of the halyard at the masthead is under half the load of the luff tension and the sheave at the masthead is under the other half. Together they add up to the same masthead compression load as a single part halyard.

Imagine a beam with a weight suspended from it. The beam caries the same amount of weight regardless of how many cables suspend the weight. The tension of the halyard is reduced by the amount of force added to the beam. The load on the beam is unchanged regardless of the number of blocks in the system (assuming for this discussion that the blocks have no significant weight).