The essence of freedom is choice. Purism would not be doing anything to prevent you from installing absolutely any operating system you like. However that does not and cannot mean that Purism can do much or anything to make the other operating system work.
“Support” means that Purism claims that it works and Purism would, within your consumer rights, make it so when it does not. Hence “support” really only means PureOS.
“Preventing” you from installing other operating systems is like the crappy secure boot path that some other computers have where, without getting keys loaded onto the mobo by the manufacturer, you can only install one operating system (M$); or like completely locked bootloaders on phones; or like keeping hardware interfaces secret and/or closed source, which intentionally or otherwise discourages or prevents you from using Linux or some other choice of operating system.
Purism doesn’t want to do any of that.
Pureboot and the Librem Key are an additional variable.
Maybe @2disbetter can shed light on whether Ubuntu 20.04 + Librem laptop works with Coreboot+SeaBIOS or works with Pureboot or works with both, and whether it is being used with a Librem Key etc.