I was in Argentina in August, 2006 when the liquid bombing plot was uncovered that eventually led to the 3-1-1 liquid restrictions. Departing EZE for ATL they absolutely did screen every pax & every carry-on item in the gatehouse. They had blocked access to the area, had several tables/agents set up and were checking all electronics as well (i.e. turn on your camera, phone, etc.). Once you were screened you couldn't leave the gatehouse without going through it all again, i.e. it wasn't like some checks that happen on the jetway during boarding.
My guess is that this new rule will play out in similar fashion as gate checks would be necessary to ensure nobody purchases anything to bring on board after the screening check point. On that flight, anyone who had purchased duty free liquids were required to return them. I had bought a couple of bottles of wine to use up some leftover currency & when I asked the agent to just throw them away he insisted I go back & return them. Though they were nice enough to hold my already-screened carry-on behind the tables so I didn't have to go through the full search again. I flew ATL-CDG 9/18/01 & don't recall that thorough a search.
As I recall the EZE check-in agents were also pretty liberal about letting pax check what they might typically carry on, because the whole situation was so new the restrictions were a moving target. IIRC at the time some carriers/routes only allowed pax to carry on a ziploc-type bag w essential documents, meds, and the like.
In other words, if the potential threats warrant screening every passenger and every carry-on item, it's likely they will...'out of an abundance of caution.'