That's not what they're saying. They're saying that the foliage causes interference with the signal getting from the camera to the nearest cell tower then back to broadcast central. Has nothing to do with all the other cell phones on the course, it has to do with the signal quality overall.
from 11 minutes ago:
PDGA Live Scoring @pdgalive
The signal is awful out here. We would like to ask everyone to turn their phones off at this time they're causing interference.