It appears to be available on a few other models as well (searching 'site:dell.com "ME inoperable"' brings up service tags for a few non-rugged Latitudes and a couple of Optiplexes), despite it not being an option on their configurators.

I wonder if this means that it's also possible to disable ME on those machines after purchase (without triggering Boot Guard)?

Dell only offers a single download per firmware version per model (or group of models) to cover all configurations, so presumably it contains multiple images (i.e. AMT-enabled, AMT-disabled, ME-disabled), and the updater just uses ME status or DMI data to determine the correct one.

If that's the case, then surely it should be as simple as manually extracting and flashing the ME-disabled image, right? That's assuming it's actually included in the publicly available updaters, and that all of the images are signed with the same key, of course.