It is not slot controlled but it is Capacity controlled by the FAA during certain times of the day and during spring, summer, fall.
Any airport is capacity controlled if it goes over the set Airport Arrival Rate based on current conditions, so that doesn't really mean anything. I work EWR arrivals — prior to 2020, it was metered either with mile in-trail (MIT) restrictions or Time Based Flow Metering (TBFM) nearly all day year-round (along with EDCTs for departures). Even with reduced traffic now, there are still mile in-trail restrictions for roughly half the day these days, and those flow controls are becoming more frequent even in good weather conditions. Every large airport in the US is capacity controlled to some degree on most days based on the AAR relative to predicted arrival numbers.
The point of having an airport be slot controlled is that it at least attempts to make the traffic more consistent and manageable. In my opinion, it doesn't work at LGA, JFK, and DCA, but it could work better if the way the slots are controlled is restructured. They still get in-trail restrictions and EDCTs, but it forcibly caps the growth and ideally makes the flows less cyclical throughout the day. UA fights to get the hourly AAR set by the FAA at EWR raised so that they can cram in as much traffic during their banks as possible — meaning they negotiate a higher arrival rate than was determined to be feasible by the Traffic Management Unit. They do so knowing that there will be added fuel expenses due to vectors for spacing, speed control, and holding due to volume.
EWR was honestly no worse than LGA, JFK, or DCA with pre-covid traffic, but if UA wants to ramp up their operations in combination with increased jetBlue service, slot controlling the airport is going to be the only option left to make traffic manageable. Prior to covid, we held for EWR on a near daily basis as it was, sometimes multiple times daily. If traffic counts increase to exceed 2019 numbers, it's eventually going to become unmanageable. I personally think United wants the airport to be slot controlled, because it will solidify their position and make competition more difficult.