Advertising has little if anything to do with the act itself. Richard Bridge might be able to confirm this, but I believe the operative word to be "organised".

For a session to be impromptu, there must be no organiser, no as it were guiding intelligence. Just a bunch of people meeting by chance, and starting a session.

It would be difficult to defend, on the basis that arriving with instruments would imply intention to perform, hence some degree of prior organisation.

Don T.