Depending on radalt for this flight mode is a problem when flying over buildings or terrain when one wants to maintain an absolute altitude relative to a common datum. If done perfectly radalt would be hammering back and forth with every chimney and tree and park and car. I'm sure they use smoothing to give something for the crew to read, but it doesn't tell how high the terrain is that the measurement is made from - it only reports clearance to the terrain when one wants clearance to other aircraft.
For TF/TA radalt is the go-to instrument, but for coordinating multiple aircraft to maintain vertical separation, barometric altitude is more suitable. The problem being that barometric altimeters are subject to a lot of measurement and reporting errors.
I am sure that GPS-RTK could be used to fix the absolute altitude with great precision, but I am also sure that depending on an easily denied measurement source on a military aircraft is not going happen.
The correct solution for operating in a civilian airspace is to use ADS-B In/Out for all manned aircraft to provide appropriate and timely situational awareness. While ADS-B is also subject to denial, it offers far greater benefit in civilian airspace over GPS-RTK in that it tells the pilots where all the other aircraft are rather than simply being more precise about where their own aircraft is.
It is clear that the helicopter crew not knowing where the passenger jet was was the primary cause of the collision.
Arguments about the error in the altimeter readings are suitable to emphasize that depending on them in a crowded airspace is a fool's choice and should have been spotted a long time ago as insufficient to provide clearance.