Which is a completely reasonable amount of traffic for one controller to handle. This wasn't the controller's fault. The firetruck received a clearance, had that clearance revoked, and either didn't hear the revocation or ignored it.

If you have ever spent time listening to LiveATC you will realize that like everyone, "tunnel ear" is a real thing - if United 1002 has received the clearance/instructions they expect, read them back, and are proceeding it can be moderately difficult to get their attention again, even with perfect verbiage.

The controller was not guilty of malfeasance, but clearing the trucks onto the runway with an airliner on short final was a mistake, no matter whatever else one could say about it.