FYI, I sent an email late yesterday to the Traffic division of the Arlington police department. I described my siting of the electric motorbike and included the same picture I posted here.
I got this reply early today from an Arlington police officer:
"So, you're essentially saying you saw/are seeing a moped on the bike path. E-bikes are allowed because they do not fall under the new definition of a "motorized vehicle" and the town has not elected to ban them from our section of the trail; mopeds still aren't allowed.
If you have a usual time and area where you see this moped on the trail, please let us know and we'll try to have someone monitor the area to gain compliance"
My interpretation is that he is stating that an electrified bike that does not meet any of the three ebike classifications, and is therefore a moped, is not allowed on the MM. If we (concerned users of the MM) can narrow down when/where this moped can be observed, they are willing to attempt compliance.
Tom