Yeah that seems to be an overly strict interpretation. If you want to go that far, why not consider hand signals and/or your voice to be a communication device??
Case book play 1.6.1 Situation E references a similar situation but with electronic signage (my guess would be holding up an iPad or tablet with a message) and the ruling states: "This would be considered to be communication with electronic communication devices and is not permitted." To me, this reiterates that only electronic devices are at issue and a chalkboard, markerboard, poster, etc. would be legal.