You may have to ask Purism directly whether either of the boot firmware choices has support for Opal.
I think in the open source world, disk hardware encryption is considered a “bad thing” because it is not auditable and other similar or related issues. For all you know the disk firmware could be implementing the identity function for encryption or using something a bit more subtle but deliberately flawed or …
I understand that if you are using disk hardware encryption in conjunction with e.g. LUKS then the disk hardware encryption doesn’t hurt.