I'm also an extra-class radio amateur, and I too am unconvinced by this article. The "tripwire" concept looks good in 2D, but I'm less convinced in 3D. Most HF signals propagate by bouncing the signal off the ionosphere (it's actually refraction, not reflection, but...), so the ideal path to the observer looks like shooting a signal straight at the horizon (that's the top of the ionosphere that's farthest from you and not shadowed by the earth). The angle of incidence equals the angle of reflection on the way down, to an ideal receive location where that point that the sender aimed at is where the receiver is aiming at. You can have multiple hops; the ground reflects signal back up towards the sky. (Tangential paper: https://www.physics.princeton.edu//pulsar/K1JT/HFTOA_1.pdf You can measure how many hops a signal is taking at any given time, and it changes throughout the day.)

So if we were in a "spherical cow" universe, you could see how this would sort of act like a tripwire. A radio transmitter illuminates a single point in the ionosphere, and a radio receiver looks at that point. If a plane flies along that path, the signal goes dark, and you know something crossed your tripwire.

The sticky bit is that we aren't operating on points, we're operating on a continuum. If you aim your radio beam at the horizon, maybe 10% of the power is 10 degrees above that, and 5% is 20 degrees above that, and so on. The signal is "smeared" in the real world to the extent that it's not a plane-sized beam that is on or off, rather it's a big circle that gets brighter or dimmer as planes fly through it. And the circle is much, much bigger than any plane. I would be surprised if you'd even notice. (That is assuming an ideal one-hop HF setup. I have never heard of people routinely operating WSPR with an ideal setup, usually people use random wires that have very little directionality to them.)

An analogy that you can try right now is getting a camera with a lens that has a large front element (I just tried this with a 90mm f/2.8 lens). Take a picture at the widest aperture. Then stick your big ugly finger in front of the lens, right on the front element, and take another picture. If you look at the second picture, you won't know that your finger was there. There is no obvious finger, or even a finger-shaped shadow. If you compare it to the other picture, maybe you will notice a difference, but maybe you won't. You have to cover a lot of the lens before you notice. (Added fun: consider the 2 minute exposure time of WSPR. Take a picture with a 2 minute exposure time, and walk through the frame midway. Can you see yourself in that picture?)

I have to imagine that using WSPR transmitters is a lot like this. If you take lots of samples, maybe a single tiny airplane will make a difference. The data is technically there. But I don't think the random transmitters and receivers are consistent enough across cycles to be able to get this data from a single sample. (Remember that WSPR is 2 minutes transmit and 2 minutes receive. Does your radio heat up and cool down, shifting the frequency of the local oscillator in that interval? Mine certainly does!)

As I was reading the article, I thought "neat, but I'm not fully convinced" until I read the part where the author thinks he can do this arbitrarily and detect any airplane at any time, and that it is conveniently done with proprietary software that doesn't show up anywhere on the Internet. That caused the alarm bells to go off. I'm going to need a lot more data to be convinced.