That was true for IBM mainframes manufactured prior to 2005. The hardware management console computer (kind of like BIOS setup on steroids) was a PC running OS/2. Models since 2005 have used Linux instead, although the management app on Linux was designed to duplicate the visual look of OS/2 (to make existing users feel more comfortable.)

I remember around 2006 or so, watching a service technician replace some faulty hard disk drives in an enterprise SAN. He opens up the front door, and out of this compartment he slides a laptop computer which runs some management software. From the look of the user interface, it appeared to be running Windows 3.1.

Raises hand I have a Multiprise 3000 and this is true for me, or would be if I could get the software that will actually bring up the mainframe hardware.

Yes, it uses ThinkPad laptops to coordinate the bootup of the machine, in the past these ran OS/2. Beautiful.