Yes, @Manuel is right, the mechanism is the same as Librem 14/Mini with AX200.
PureOS uses the Debian kernel for x86, not Linux-libre. The kernel code including drivers is all free and open source, of course. The card firmware does not run on the x86 CPU, it runs on the card’s embedded processor. This is true regardless of whether the card firmware is held in a file or in a flash ROM chip.
PureBoot’s Firmware Blob Jail just provides the device firmware files for OSes that do not have them (like PureOS), they will be in /run/firmware if you want to see them. The OS kernel passes the contents of that file to the device.
What Linux-libre does is remove the ability to load any device firmware (for most devices, including Wi-Fi cards). Linux doesn’t contain device firmware to begin with (it’s separate), but Linux is able to load the device firmware if it is provided. So if you run Linux-libre, and you choose to provide Intel Wi-Fi firmware, the kernel refuses to load it.