In Example 4, what is the idea behind substituting G7 by Bdim? (Isn't the tritone sub Ab7?)

@emilernebro Thank you for the prompt and detailed answer! This is very helpful.

I very much hope to see more content like this lesson on ProGuitar.

Best wishes,
Tom

In Example 4, what is the idea behind substituting G7 by Bdim? (Isn't the tritone sub Ab7?)