If the dead cells were shed, I might agree! But they're not.

This is like counting the dead skin cells that are still attached, which (sure enough!) does count toward animal biomass.

It serves an important structural purpose for them. Similar to corals.

A lot of the wood is used for transporting water - vital to keeping the trees alive high up in the branches. And it is structural - so should we subtract 70% of the bone weight from a mammal since it is non-living structural support?