Yes. Depending on the frequency(s), the spaces between cuts on the waveguide vary. But it's basically just open space in the cable/waveguide letting frequency leakage. Coax or waveguide (hollow cabling) can be used.

This project uses leaky feeders? https://en.wikipedia.org/wiki/Leaky_feeder

I must have grossly misread TFA. The way I read it, he is just using an existing coax cable as the wifi RF in/out on 2 APs. Exactly as one would use a commercially available MoCA bridge.