Yes, it's almost certainly a single Hall sensor. In it's pure form, a Hall sensor is an analog device. The stronger the magnetic field, the higher the output voltage.
Since we often want to use them as switches, many sensors carefully digitize this signal into an on/off output. However, you can get analog ones. The driver in this light is built to take the analog voltage from the sensor and control the output current from that.
For an electronics designer, that's not hard to do. But if you're not prepared to design and build your own driver, you have very few choices. Some of the LuxDrive drivers accept analog inputs. Those and my IS1006 and 1011 are the only flashlight-size drivers I'm aware of that are designed for analog input. I don't specify mine that way, but they are in fact analog input.
Taskled's H6CC, Hyperboost, and Hyperbuck drivers accept 'potentiometer' input. I'm not sure they would work with a voltage input. I'd be inclined to think not, but you'd have to ask George to be sure. His other drivers definitely wouldn't accept analog input. I checked pdb-components.de and none of their drivers accept analog input. Their LedStripe dimmer accepts analog inputs, but it costs more than the drivers that accept analog input, and it still needs a driver!
Another problem is that the Hall sensor naturally has an output voltage that's half the supply voltage when no field is present. The voltage increases for one polarity of field, and decreases for the other polarity. All the analog sensors I looked at maintain this characteristic with their processed outputs. Unfortunately, you want zero light when there is zero field (or perhaps a small non-zero field). Some additional work would need to be done to achieve this performance, unless you can find a sensor that has zero output at zero field. I didn't look really hard, but I think it's unlikely you'll find one.
I did find some programmable sensors that could be programmed to work the way you want, but buying a programmer for a one-off build is probably cost prohibitive. You might find that you can buy them pre-programmed for this behavior. I didn't pursue that option because it wasn't obvious to me how to do so.