and uses a different BIOS.
You seem to be the one expressing confusion! ;-)
Look at a typical smart phone or tablet - it probably has a power button, and a volume rocker. Hold the rocker in one direction as you power it up, and you enter engineering mode. From a software point of view its a fairly trivial exercise to check the status of an input latch to read the status of the volume rocker switch during the boot, and take different action from normal, such as entering an engineering mode. This behaviour is common on phones etc and has been for years.
Its not at all common on (traditional x86) laptops.
Now look at *most* laptops - no dedicated volume control. If you want to alter volume, you do so with a sequence of key presses like Fn + and arrow key or a number key etc. If you try to hold that combination of keys while powering on, the likely results are either no effect at all, or some daft error like "keyboard error, press F1 to continue".
On a laptop the "engineering mode" is usually simply the BIOS setup screen, and there are a number of common ways already well established for getting into that. All typically accessed *after* primary boot and once software is configured to be able handle keyboard events.
On a typical laptop this is true, but not necessarily true on phone style hardware. I am sure you are aware that reading the state of a hardware button is significantly less complicated than displaying a soft button on the phone's screen, and getting input from the digitiser to resolve a touch event. You need significantly more operating system up and running to do the latter.
While that's mostly true, there is a significant hardware difference that makes detecting keyboard keys held at power up impossible even for the BIOS.
The keyboard switch matrix is not accessible to the CPU and hence the BIOS, and is instead read by a dedicated keyboard decoder, which sends "key up" and "key down" event scan codes over a serial link (which in turn may often be tunnelled through a USB protocol stack these days).
So before the keyboard can be "read" the BIOS will need to do loads of initialisation including possibly setting up USB host adaptors, configuring the protocol stack, enumerating each USB device and then finally sending initialisation messages.
Once that is done the BIOS can start to interpret the stream of bytes from the keyboard. Processing the stream also needs the BIOS to maintain a concept of "state" since most codes do not include sufficient information in their own right to allow full interpretation.
So say your laptop uses Fn + F5 / F6 for volume up and down. The scan code sent for "F5 down" (0x3F) is always the same regardless of whether Fn is held - its the sequence of arrival that tells you its F6 and FN together. That's another reason why you can't detect a static combination of keys held at power up - even the keyboard decoder would not know what sequence of keys you pressed before power was applied.
Hopefully you now understand that from a software point of view, a physical volume rocker is a very different beast from a normal keyboard key.