I should also point out that this has been the process in Asia (and some parts of Africa and Latin America) for over 50 years to my certain knowledge. The current WHO guideline, still in place, dates to 1995 but that formalised what was already in place via the ICAO. It's a specific requirement, in many cases, for the passengers to be sprayed, you can't do it before boarding (though that is allowed in certain scenarios). From a USA perspective it is quite a short list of countries, India and Panama are the two that the OP may have encountered before.