As I understand it, transmission typically takes modulated RF and boosts it. Its applying brute force to a pre-shaped signal as the "carrier". The RF has to come into the amplification stage over a waveguide. Typically it's being sent from somewhere else, down a landline or local point to point microwave link. If you can "flood" the amplifiers input with a stronger signal generated locally, with either a path into the RF waveguide or a strong directional antenna and a receiving dish for the microwave signal, it will latch on to it, and amplify it too. If it's stronger at source, it's stronger post amplification/transmission. Being stronger at source is easy if you are close.

It probably does not work as well on a digital stream, or a multichannel, agile frequency hopping signal but these tend to be short range. These numbers stations have to work with old, crude and remote receivers and with more traditional (deliberate) interference from distant sources so they are more prone to being subverted by a motivated local phreaker who can inject a strong signal from e.g. a car parked close by and then run away.

You would think military communications are 24/7 guarded but perhaps in some economies people assume distance from the urban area is protection. It doesn't stop motivated attacks. By the time you find out, they've taken the Max Headroom VHS and their radio rig away in an anonymous van.

Maybe this attack is smarter? Doing something less brute force and not just flooding the last-stage RF amplifier?