No. UEFI mandates GPT. GPT does not mandate UEFI.
Please read the post above where my Intel UEFI-enabled server boots the Proxmox partition. You keep ignoring that. Intel wrote the specs, Intel made the server, Intel made the firmware, Intel boots the hybrid drive without a hitch. The constant here is the drive partition format. The variable is Intel firmware (NUC vs S2612UR).
You keep saying the same thing with different words all the time, but you fail to understand simple things:
1. UEFI requires GPT drive. GPT drive does not require or imply UEFI
2. there are both UEFI and non-UEFI machines that boot successfully so the drive format is not an issue
3. there are machines that do not boot. To me it looks like the firmware is buggy, following your logic: GPT means UEFI only (which is not the case, not mandated by any spec)
4. NUC has Windows as the only supported OS. That means that probably it makes some assumptions (see 3)
I understand that you're upset about your specific configuration failing, but you cannot ask to change a system that works and fits most of the people to make a NUC happy. If you search about NUC boot problems with Linux you will find lots of pages and discussions about firmware updates that sometimes fixed, sometime didn't fix the issue.
If you have a non-working configuration, you have the workaround to install Jessie and then Proxmox on top of it. That is a luxury that you don't have in other virtualization platforms.