Why URDF-related error raises when loading XML mujoco file in pybullet?

Question

I'm trying to import xml file in pybullet which has no problem on loading in mujoco.

Error message is that

b3Printf: b3Warning[examples/SharedMemory/PhysicsServerCommandProcessor.cpp,3584]:

b3Printf: URDF file with multiple root links found

Even though not using URDF, why is this error raised, and how can it be fixed?

Test file in mujoco. Worked well.