You can find the following clarification in Exp. 5:
https://wikicarpedia.com/index.php/Abbey_and_the_Mayor#cite_note-9Any meeple, including the mayor, that is placed in a city is considered to be a knight.
So the mayor would also be considered for Message #6 - 2 Points per Knight.
Note that counting meeples is not the same as counting their majority votes (or strength) to decide the majority in a feature.
* A regular meeple is one meeple with strength 1
* A large meeple is one meeple with strength 2
* An abbot is one meeple with strength 1 (it can only be placed on a monastery or in a garden)
* A wagon is one meeple with strength 1 (it cannot be a farmer)
* A mayor is one meeple with strength equal to the number of coats of arms in its city (it can only be a knight)
* A ringmaster is one meeple with strength 1
* A phantom is one meeple with strength 1
So all meeples except abbots can be a knight and will count as 1 knight each for Message #6, no matter their strengths. Note that meeples placed on monasteries within a city will not be counted as knights, since they will be monks (in this case, you may find n abbot within a city but it is acting a s a monk not a knight).
Here you are two examples of monasteries within cities from Exp. 3 and Exp. 8: