It will ship Pureboot by default (Coreboot+Modified HEADS) on all Librem 11 devices. To be specific it will ship Pureboot Basic (Introducing PureBoot Basic – Purism). Which allows to use the device without a Librem Key.
If you have a Librem Key, you can enable the full features of Pureboot in the Pureboot menu. Thus enabling the tamper detection feature of Pureboot+Librem Key.
The USB ports of this first iteration of the Librem 11 are USB-C, but with an USB-A to C adapter you can use the Librem Key.
It should be possible to provide a different bootloader option (Coreboot+seabios or others).
But Pureboot basic provides the same features as seabios: Hardware information, Booting the device, selecting to boot from a USB device, but adds to that (among other things) a graphical interface, which our previous coreboot+seabios did not provide. And also a recovery shell for different operations if needed
And has the option to enable full Pureboot if you prefer.
EDIT: A coreboot+seabios option already exists. And you can flash it with the usual scripts.