System76 disables ME by setting the HAP bit, this ability has been around for a while and there is a pull request for it here:
for me_cleaner.
There’s a couple versions due to the partition layout changes in the newer firmware. Entirely removing ME the way they do in the current Librems may be more complicated for them to accomplish on the newer chips running the newer versions of ME due to the partition layout changes. I won’t be terribly surprised if the next release of the Librem doesn’t have ME entirely gutted or isn’t on a 10th gen chip due to potential problems with gutting the ME on the newer firmware versions.
System76 is just using re-branded Clevo’s so they’re not manufacturing the laptop the same way Purism is. Instead they customize the firmware and have their own Linux distribution called Pop!_OS.
They’ve been around for a while and from what I recall they worked out some issues with hybrid graphics in Pop!_OS when most distributions were still having problems in that arena when it first came out.
I would expect good firmware/software support from System76, but not as much on the hardware front like Purism is doing.
They’ve also added EFI boot capabilities into coreboot by integrating EDK II into the coreboot firmware for their laptops.
So definitely some differences, I would expect System76 to be able to keep pace more rapidly with newer processors due to the way that they have approached providing support for Coreboot on a few of their laptops.
The thing it is currently missing that Purism has is PureBoot.