are dendrites why so many people are now burning down their apartment/complex charging their scooters

is LiFePo4 immune to dendrite shorts? Or do they happen just don't burn?

There are a number of reasons why lithium batteries may catastrophically fail and catch fire. Dendrite shorts is one, another reason is poor alignment of the layers during assembly, allowing for eventual shifting of layers leading to internal shorting. Another is conductive or sharp debris getting into the battery during manufacturing, and after a while the anode/cathode separator getting pierced by the debris. Lots of reasons!

dendrites are not really a significant problem in popular batteries. It's associated with lithium metal, vs lithium in normal batteries is in the form of salts. Solid state lets you use metal, which is much more energy dense since you don't need the salts.

The most common lithium battery failure mode is that you have a hole in the plastic separator between the +/- sheets inside the battery, which shorts and causes a hotspot that eventually starts a fire. Dendrites cause the short by growing across the gap. In normal batteries it is caused by a manufacturing defect. The outcome is pretty similar.