A Russian space nuke absolutely makes sense in military terms:

A high-altitude Nuclear-EMP creates an electric free field incident powerful enough to damage any microcircuitry onboard satellites (see: Starfish Prime Test), especially modern electronics due to increasingly thin structures (therefore electric breakthrough occurs already at relatively low voltages). We are hardening military systems against such EM incidents by shielding, e.g. on a ship using its hull as a Faraday cage. This can't be done on a satellite due to weight constraints.

First, a space nuke would disable recon satellites, both RF (radio frequency) and EO (electro optical), therefore making command blind. Second, it would disable GPS, therefore severely reducing effectiveness of ballistic and cruise missiles guidance (inertial guidance is drifting over time, losing accuracy over longer distances). Third, communication satellites (military but also commercial such as Starlink) would be disabled, leaving troops without orders in time of crisis. Finally, radars are cluttered for days by charged particles bouncing along earth's magnetic field lines, emitting EMI, making air defense and air operations almost impossible.

It would instantaneously nullify most of the US most powerful military assets. It's the ultimate preemptive strike weapon, so it's clear why they would be concerned.

Of course no WW3 will break out, but instead they will use this to beg for more money (as always) to spend on useless high-altitude air defense systems (mainly LMT & RTX, over years to come).

— from a commenter