I can not find any references to other BIOS settings related to CSM support in this specific case that indicates you can still BOOT a CSM OS.
You see next to the BIOS having CSM support you must have drivers that support the new hardware.
I am not sure what they mean there - or where it says it only supports 64 bit operating systems.
The manuals don't give very much information on CSM - or in some cases any information at all.
With my AM4 socket MSI motherboard the way you choose whether to boot UEFI or CSM mode is through the BIOS boot menu - I suspect it is the same with AM5 socket boards. Basically each media that is attached when the machine turns on is listed twice: once with a UEFI tag and again without the UEFI tag. If you pick the media with the UEFI tag you boot in UEFI mode, otherwise you boot in CSM mode. This isn't described in the manual. With the other two manufacturers I had to look on forums where people posted about problems booting CSM or UEFI to find out that the board supported CSM.
With the choices between UEFI and CSM, MBR verses GPT, and how UEFI vs CSM affects the capabilities of both SNAP and Panorama, along with DOS and/or Win-OS/2, setting up a system is now a very complex undertaking.
Making an educated guess based on what is written it means you can for example use PCI cards that still have legacy support. Such as an old video card.
As I typed before as time moves on supporting CSM for old operating systems makes less and less sense. This because the mainboard vendor has to have drivers from the companies that make the chipsets, for example. video, audio etc. I would be surprised if all of these mainboards could support Windows 7 for example. Hence the reason vendors will just dump CSM support.
Unless you buy a legacy refurbished system or with a industrial mainboard (but these can be expensive).
When it comes to the choice between GPT versus MBR on UEFI boot. Always grab GPT when booting an MBR system. If you currently have a working MBR just jeep it. But in my experience MBR is fragile and especially LVM from OS/2. The genereal rule of thumb is you should ONLY use disc tools that run under OS/2 to edit your disc. Not doing so can bring you into problems.
Especially if you want to setup a dual boot system. The design of GPT is simply more modern and less fragile. Other people who have installed a dual boot with Windows 10 or Windows 11 on GPT I think will confirm it was never easier.
The headache is indeed with SNAP. Then you need a working mainboard with a CSM.
Roderick