I ran into the same problem on my libreboot x200. Solution is pretty simple. Basically the ISOLINUX folder of the PureOS ISO only has an EFI boot option. There is a legacy grub in a separate folder but that is broken as it doesn’t point towards the location of the kernel.
To boot the installer do as follows: In the libreboot boot menu select “search for grub2 configuration on external media”. It will drop you into a blue screen with two " live" options. Highlight the first one and press “e” to edit it. The file paths there for “Linux” and “initrid” do not point to where the files are. Add /casper/ on each line before the file name. Press F10 to exit and boot and you will land in the installer.
Should be an easy thing for Purism to fix in the ISO so posting in the hope that they can do that. Given that librebooted computers is the only hardware FSF certified it seems important that PureOS, one of only a handful of distros to be FSF-certified, can install easily on them.