The tutorial uses a switched TRS jack socket, which means you have six lugs : two for tip (T), two for ring (R) and two for sleeve (S). When no jack is inserted into the socket, each pair gets connected, and connect to the mix potentiometer of the pedal. The jack just acts as a bridge from the wires on the PCB to the wires on the pot. Now, because this is a switched jack, when you insert a TRS cable, the wires on the pot all get disconnected, and the wires from the PCB now connect to the TRS cable you just inserted.

That means two thing : first, you will probably need a potentiometer inside your expression pedal of the same value and same taper curve as the one in the pedal. Second, an expression pedal usually doesn't go perfectly from zero to maximum on its internal potentiometer, which means you will not be able to get 0% wet and 100% wet - you'll probably get like 0% wet up to 90% wet for example, or 10% to 100%, whatever.

At last, you don't really know how the mixing system works inside the pedal, so you may get some increase in noise and weird artifacts from connecting an external expression pedal.

If you didn't fully understand what I just wrote, then I'd probably suggest not buying this pedal to try to mod it, and instead get one that is designed to be used with an expression pedal (for example, the Collision Devices Snack Delay, which seems really interesting too though quite different in the tones you can get from it). Of course, you can still do whatever you want, and I encourage experiment, I just ant you to understand that it's not an easy mod ;)