They're not Controlled by Israel. However that group is facing a nation-state surveillance and potential threat level. Making things easier for them by using a commercial platform is downright stupid. Of course I suppose most of them to be completely ignorant in those matters, but there ought to be a few people versed in technical things in the group.
Also using, say, Signal, won't keep Israeli spooks out for sure, but it will at least make it a bit more complicated for them to be a nuisance.
P.S. tagging you as anti-american conspiracy nut, just in case.