The problem with using some of the course topics or content as material for the lecture is that it usually results in the candidate simply rhyming off rote what someone else told them during the course and they are doing nothing more than copying what they have been told during the course.
A unique unrelated topic requires them to use what they should have learned about preparing and giving a lecture; preparation, classroom aids, timing, lesson plan (that is not the same as the 100 previous candidates) questioning, feedback etc.
I think that it is lazy to cover two elements of the test in one part i.e. trying to grade the ability to give a lecture at the same time as checking some area of required aeronautical knowledge.
If one wants to check their knowledge then there is plenty of scope for questioning.
Thereby it could be said that the examiner leads by example.
Have you ever watched a "natural" teacher give a quality lecture about say the solar system. The realy good ones will have you captivated, make appropriate use of aids, use appropriate expressions, language, body language and even as you listen / watch you are aware that you are learning something.
Droning on with 100% accuracy about the stalling characteristics of a PA28 using the standard school material in the standard school format tells you nothing.
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Of course, the briefings for the flight lesson must be appropriate but I think that letting this new teacher run with something that they know well and can make interesting gives the examiner a chance to glimpse what could be the standard in a few months or years time when the candidate is as relaxed with the aircraft knowledge as they are with content of their chosen lecture.