I think the author is too quick to dismiss the impact of the plugboard.

> IC attacks only the rotor settings [...] Once you’ve found the right rotors and positions, you can solve the plugboard separately using frequency analysis.

So two steps: first solve for rotor settings with IC analysis, then solve the plugboard.

But IC analysis can't find the solution, just propose a lot of candidates among which a human can spot the correct one by looking for intelligible German text. And how can you spot intelligible German text if some letters are swapped by the plugboard?