They don't advertise the wattage of the heating element. At least I couldn't find that on their website. Without knowing that, you have no idea how long it would take to boil water. If it's not a powerful heating element, it could be like putting a pot of water in your oven set to 212 degrees. You would be waiting a LONG time for that to boil.
They have 12v immersion heaters:
https://www.amazon.com/Travel-Chocolate-...8SZA9R7VMG2FN4TBut I would use great care with one of these. First, your car's cigarette lighter plug may not power the thing (you possibly/probably could blow a car fuse). You might need to hook it directly to the car's battery, not via an outlet.
150 watts (as this immersion heater advertises) at 12v equals 12.5 amps. You might melt those thin-ish looking cables on the device at that amperage. And you might need an oven mitt to handle it. Is your cigarette lighter plug wired/fused for 12.5 amps? It's probably fused lower than that!
Notably, this immersion heater doesn't declare itself to be "UL Listed". I wonder why...
The original mug discussed in this thread advertises a separate 5v LiIon battery to power it. If the mug's heating element is, say, a measly 100 watts, lordy lordy, that would require 20 amps! You wouldn't even need a heating element to warm your coffee at that level. Drawing 20 amps out of one of these small LiIon batteries would heat it up so much that the battery alone would heat your water. No need for an external heating element. Just short the battery and drop it in your water directly.