Piano shares space with the whole arrangement
A piano note begins with a short percussive attack and can sustain across a broad range. Those features may overlap with drums, bass, guitar, voice, and reverberation. A separator must decide which shared sound belongs with the piano, even when the mix does not contain a perfectly clean boundary.
The training label changes the task
Common four-stem research sets group vocals, drums, and bass separately while placing remaining instruments together as other. A dedicated piano output asks a more specific question than that four-stem task. Results from one grouping should not be presented as evidence for another.
Audibility is not a score
The reference excerpt on Labs raises the piano level so quiet details are easier to hear. That is useful for study, but the gain change prevents a fair level-based comparison. A model evaluation must disclose such adjustments and keep the untouched output available.