OK, I found a Lenovo doc:
Can I disable Switchable Graphics?
Yes, there is a BIOS setup option to disable Switchable Graphics. In the BIOS setup utility (press F1 immediately after powering-on the system, while the ThinkPad logo is shown),
enter the Config menu, then enter the Display menu. Change the Graphics Device from Switchable Graphics (the default) to either Integrated Graphics or Discrete Graphics, whichever you prefer.
Switchable Graphics Frequently Asked Questions (FAQ) - ThinkPad R400, T400, T500, W500
There was a confusion in my mind between the
BIOS, which one enters with F1 during boot
Intel Management Engine BIOS Extension
listed in the above, and which one can enter too, during the boot., with Control-P.