I don't understand what makes it hidden if anyone with the right equipment can pick it up. That's like calling X-rays hidden because most cameras can't pick them up.

True. I think the novel point is that on average, the emitter just isn't emitting. Normally, you can have a very simple piece of equipment that can pick up the fact that a signal source is emitting something, but then you need to get a more specialised piece of equipment to actually collect and decode the transmission. This just raises the stakes to having to have the specialised equipment to see that there is a transmission at all.