The U.S. declined to provide a security "guarantee" because that would have amounted to a backdoor accession to NATO, and the U.S. could not grant that unilaterally. (An example of the U.S. honoring a prior commitment, by the way.) A "guarantee" would have required the U.S. to commit its military forces to the battlefield if Ukraine was invaded.

The U.S. instead provided a security "assurance" which was understood to mean a level of support short of U.S. troops on the ground. And in fact that is what the U.S. has been providing: intelligence and material, but no U.S. forces.

In other words, the level of support has been commensurate with the agreement signed. Until today, apparently.