Yes, I used exactly what was spec'ed in the article (I had a pre-existing SSR so that was different) and YES, there can be issues sensing enough current if you aren't drawing enough current like one might in winter time running a variable speed pump at a very low speed. I experimented with my VS pump and I can render the circuit useless (i.e. not drawing enough current to sense) running the pump at a low rate. I don't recall where the RPMs were but I'd hazard it was 1k or less and the current sensing wouldn't work.
The current-sensing transformer in the diagram is the issue as the windings are 1:500 so it needs a healthy current input to output enough to allow the relay to work. So this circuit may not be ideal in the winter if you are running your VS at a very low rate. I personally don't see this as too much of an issue when I close the pool for the winter since I cover my pool, there is no need to run the booster pump and the chlorine demand is super low, too, so I could just manually dose.
That said, I plan to mess around a bit with the design and see if I can get the switch to work at low pump RPMs (low current) to make it better for our situation.