It's the underlying system. You can run tiny bare metal programs in your boot sector that simply call bios functions to output to the screen and write to disk and beep the speaker, just like you can call functions to the windows or Linux kernels to do the same.
Although granted the only functions called these days are to set the CPU to protected mode and goodbye BIOS until the next boot.
hmm. it's a good point. it seems to me though that that just isn't what "operating system" refers to, since these programs can't actually manage the system in a meaningful way or do anything you'd really expect from an OS.
But circling back to OP's screenshot it only mentions that it's part of the OS. A pretty small part these days compared to DOS, but it's still doing stuff.
I guess it comes down to whether you consider all the initialisation/bootstrap/hardware configuration routines to be part of an operating system.