pnpm add -g node installed Node correctly, but the node shim in the pnpm bin folder was wrong. pnpm copied the text of a relative Homebrew symlink instead of the resolved path. pnpm dlx/pnx tools that start with node then fail with node: not found.
Check the shim and the .pnpm-shim-v1-node-target file. Fix the shim or put a real node on the PATH.