Booting Intel NUC from a SD card

Solution 1:

What exactly would "USB-like" mean?

That IMO is a misleading choice of words.
There are no alternate formats that makes a SD card more or less bootable.
Those articles also conflate a (bootable) device and (bootable) media.

The built-in SD card connector (of the NUC) is probably attached to a MMC/SD_card controller.
This MMC/SD_card controller (with installed medium) is a device, but apparently not a bootable device in this NUC.
(Technically there is a difference between MMC and SD cards, i.e. the specifications are published by different organizations.)

An SD card is simply a medium (i.e. singular of media) for data storage. It does have internal electronics, but that is only to provide a PC-compatible sector-based interface instead of its native NAND flash interface of erase-blocks and pages.
By itself the SD card is not a device. It must be inserted into a connector/socket/slot, and accessed by the CPU with suitable interface electronics, e.g. an SD_card controller or a USB card reader/adapter.

"USB-like" would mean to not use this built-in MMC/SD_card controller of the NUC , but use your SD card in a USB-to-SD_card adapter (aka USB card reader) (which will emulate a HDD) plugged into a USB port.

What standard does this relate to?

You will be using your SD card (in its USB card reader) just as if it was a USB flash drive.
USB flash drives emulate a removable hard disk drive.
HDDs for PCs adhere to the ATA (aka ATAPI) specification.

What should be the file system, partition structure, what should be in MBR?

You would need to use a hybrid ISO image that is writable to either a DVD or USB flash drive. You will be using your SD card (in its USB card reader) just as if it was a USB flash drive.