Looking at the manual, it appears the device sends either 20-100% as the battery level, or 255 if the battery level is "low," the threshold for which does not seem to be configurable. This information appears to come in on the standard battery report (just with a level of 255, the max--not sure if this is standard, but it's common on devices I've seen that do this).
Many Hubitat drivers will take that information and parse it into a level of 0 or 1 -- so the information is already usable, even if not easily distinguished from a regular battery report. (This is based on observations of similar devices, not this exact one.) But for this device, something less than 20 doesn't appear to be something the device reports, so anything less than that is probably something the driver is doing in response to this state. If you've observed the states when it's actually reporting "low battery," I would expect it to do something like this. If not, a driver certainly could be written or modified to do so.
Ultimately, this is still dependent on the infamously unreliable battery reporting from these devices, BTW, so in case you're expecting it to be better than regular reports, no matter how the information gets parsed into events, I wouldn't. 
