Surely this is only true to a minimal extent in world where you can already buy timeservers that support multiple independent time sources out of the box (multiple satellite Galileo,GPS etc. + terrestrial radio e.g. DCF77).
IIRC if you're dead serious and money is no object, all the major national time labs also allow you to run a private leased line to their facility for direct checks.
As of now all GNSS constellation are somewhat spoofable, especially if you only want to drift a clock a few milliseconds late... And a lot of timeservers are still GPS only so the attack is pretty easy to perform.
The private line idea is maybe doable (but which protocol? NTP doesn't offer the needed precision), but as all substations would need one, my guess is that eventually money would be an object...
IIRC if you're dead serious and money is no object, all the major national time labs also allow you to run a private leased line to their facility for direct checks.