@jamie - thanks! Your solution worked for now, although I don’t think it will be what the Purism devs should replace the site instructions with.
I know the microcode is proprietary, but I don’t want any other proprietary code on my computer, and I have concerns that adding those repos opens up opportunities for other proprietary code to be installed via dependencies etc.
Also, after running sudo apt-get update and sudo apt-get install intel-microcode (according to https://wiki.debian.org/Microcode), I had tons of more packages besides the microcode, one of which caused an error with an overriding grub configuration file, and many of which were left not upgraded, so ran sudo apt-get dist-upgrade.
There might be a better way to do this