`!=` doesn't seem to be the exact inverse of `==`. To prove this I added
the following to the udev rules. On a system with the community firmware
both COMM_FW and NOT_COMM_FW were set to true, with the quectel firmware
only NOT_COMM_FW was true.

ATTRS{serial}=="community_fw", ENV{COMM_FW}="true"
ATTRS{serial}!="community_fw", ENV{NOT_COMM_FW}="true"