Partially true. The element in the bulb is sealed, but when turning on a light you will get a tiny spark when the parts inside the switch make contact.

I asked a friend that was a police officer in a city where there were lots of drug busts the question why in movies (and real life) the police don't switch on a light when entering a home to search.

He said that in quite a few drug houses they either booby trap the light switches as well as fumes created from meth production (and other drugs) is flammable enough to ignite. They go through training that shows that a light switch can ignite the fumes if there is a high enough concentrate of flammable/explosive gas.

The electronics they use (radio/flashlight) have been certified to not create sparks when in use for this reason.