We aren't the only ones with launch detection and NUDET capabilities
Although our capabilities are probably more comprehensive and robust than anyone elses, we don't have a monopoly on knowing when something has launched or if a nuclear device has detonated. You couldn't keep something like that secret. A non-nuclear high EMP weapon like those claimed to exist in Popular Mechanics would do a lot of damage to softly shielded satellites but most military satellites are heavily shielded. You'd also have to get reasonably close to the target - which not only requires a detectable launch but some maneuvering, too.