There's a narrative sense of some language, and a crunchy rules mechanic sense. Sometimes it's hard to tell which is which.
Narratively, strong spirits are effectively immune to mundane weapons. But in a rules sense: no they're not TECHNICALLY immune.
The way it all comes together is a little confusing.
Materialized spirits are "narratively" immune. See the Materialization critter power on pg. 225. Among other things, it says that critters gain the Immunity to Normal Weapons critter power while materialized.
That means reference page 224, where Immunity is covered. Immunity means you have Hardened Armor versus what you are immune to, with a rating equal to essence. The Immunity critter power continues on to page 225 where Immunity to Normal Weapons gets some special rules. But you now also need to look at Hardened Armor, on pg. 224. This critter power makes DVs equal or less than the rating of the power deal no damage, and gives auto-hits on the soak test on DVs higher than the rating.
In total, this means:
While Materialized, spirits ignore damage from mundane weapons when the DV is equal or less than their rating (and essence = force, usually). This means high force spirits are very, very hard to hurt. But not technically impossible.
EDIT: Damn, you're quick Xenon.