What is purpose and meaning of "Propositions" in PhD thesis?
This is indeed related to customs in Dutch academia. Precise practice and regulations vary between universities, but usually there is the opportunity (or even duty) to provide this list of propositions, usually distributed as an inserted leaflet with the thesis. The propositions themselves vary from serious scientific statements (typically related to the content of the thesis), to more "humorous" statements like the example you posted. These statements can be part of the defense, i.e. the committee can ask questions about them and the candidate has to defend this thesis. In practice (insofar they do not relate to the thesis itself), this only happens when the committee runs out of "real" questions. This can happen because for the defense in the Netherlands a fixed amount of time is prescribed, which cannot be shortened.