They do throttle, and quite low compared to other comparable engines, but they still cannot throttle an engine below 1 TWR when the stage is near empty. Meaning that they cannot hover a stage, either the engine is on and the stage is accelerating upwards, or it's off and it's accelerating downwards. (And you cannot rapidly turn engines on and off.)

So they need to "hoverslam", that is, arrive at the landing pad rapidly decelerating so that their altitude hits zero just as their speed hits zero. This was thought to be very hard, but I don't think SpaceX has lost a stage due to estimation failure there. It helps that there is significant throttle range and fairly rapid throttle response on the engines, so they can have some slack. (Plan to decelerate at 2.5g for the last ~20s or so, with the ability to do anything between ~1.5g to 4g, so you can adjust throttle based on measured landing speed.)

Their Superheavy has more engines, allowing them to bring the TWR below 1, enabling hovering.

No. SpaceX's Merlin engines use a single https://en.wikipedia.org/wiki/Pintle_injector which has excellent throttling capabilities.