No, it can not be liquid tight without sealant. The tapered threads still have helical leak paths at the roots and crests of the threads, because you can not cut a thread precisely enough that will contact in those corners at the same time that the thread faces meet.
Exceptions: plastic fittings (nylon, etc) can undergo enough plastic deformation to extrude into the thread roots to seal. A brass fitting is ductile enough that if it is way over-tightened, it may deform enough to achieve a seal. In doing so, you run the risk of splitting open the casing of the mating part.
The challenge here is that the floscan sensor has a very small internal passage with a little turbine in it. Any debris at all can obstruct the fuel flow, and/or interfere with the free rotation of that tiny turbine.
There has been at least one documented case of a tragic outcome from fuel obstruction in a fuel flow sensor, ( I believe from teflon tape, but I could be wrong)
What I did, and everyone can take this or leave it, it is simply what I did, was to use a very spare amount of teflon paste on the upper part of the threads, leaving a couple of threads completely clean. The hope was that whatever squeeze out of paste would mostly migrate outward, not inward, and even if a little bit did, it would move into those few bare threads and no farther.
After 500 hrs, no issues. I have not/can not look inside to see the actual state of intrusion of sealant into the sensor, but it works fine.
If the manufacturers of the sensor really want a liquid-tight connection with no sealant, they would machine a female "straight thread plus o-ring" boss, often called a JIC straight thread or SAE straight thread fitting. There is an AN spec for this and fittings are readily available. The fittings would have AN 37-degree flare male attachment on the other end, to receive a flare and B-nut. The cost difference between cutting a female pipe thread and a female AN o-ring boss thread is pretty small. It's just good engineering practice. Look at many many high pressure hydraulic devices to see how good connections are made that avoid use of sealant that can foul the internals.