just as an aside, he's separately and also referring to the deal between UK and France where the French agreed to accept Greenwich being the Prime Meridian in exchange for the UK adopting the metric system.
The importance of the meridians was determined by the publications, e.g. nautical almanacs, which were issued by the astronomical observatories located on those meridians.
Prior to the establishment of a unique origin for meridians, the navigators were divided in those who used nautical almanacs from the Royal Observatory of Greenwich, those who used nautical almanacs from the "Observatoire de Paris" and so on, for almanacs from other less important observatories.
All observatories published coordinates based on their own meridian (which was precisely where their meridian telescope was located).