If you first track down the location of the access point, then you know the directional antenna is pointed at you. From there, the direction finding is easy.
I think a major part of the rant is that Tor isn't provably secure against yet-to-be-discovered attacks, and several attacks against Tor have been discovered (and fixed) over its history.
Right, but the idea behind direction finding is that you can triangulate the position of the transmitter. If you just have a bearing you have to search over a large area -- these links could be many kilometers long.
Not saying it's impossible, but using highly directional antennas makes direction finding much more difficult.
I think a major part of the rant is that Tor isn't provably secure against yet-to-be-discovered attacks, and several attacks against Tor have been discovered (and fixed) over its history.