IIRC the old text explained the straight taxi requirement was to avoid brake / NWS jerk during elec transient as the other engine GEN comes online.
Perhaps the additional hyd load from NWS command and likely braking during the PTU test could invalidate the results.
Smells like an in-service event avoidance SOP. PTU fault during taxi,.... thank you but no, thank you. ☺️
All speculation, with Y Elec running why should there be trouble?