This is one of main advantages for them yes. I have also been in large protest environments where they fry cells w internal batteries. Not to spook people but I also had one fried covering Q in early '18. I've stopped buying internal batteries :)
When it gets fried your phone stays on but your screen goes dark. It then stays on until the battery is completely drained and will never take a charge again. Not sure how they do it but let's just say I' ve personally seen this exact thing happen to a dozen or more targeted people.
Ir regards to protecting yourself from DARPA or other agencies. You can't You can technically root a phone and install certain software to make a call while connected to a mesh grid but who is around a mesh grid outside of Oakland(if still up w Sudo Room and NoiseBridge)? Also another point on that. If you were an agency looking for someone with intentions they sought to hide wouldn't you isolate the phones acting suspiciously? Like installing privacy apps and rooting your phone. I would look there first. The hardest people to find are the ones who act normal and get lost in teh noise of the digital world. ie talk on xbox live, creating google drafts etc..