Officers at land border crossings have the same information as those working in airports, but they weren't given a mandate to put the screening question to travellers. Their information comes from the Public Health Agency of Canada.
We work with our American counterparts to ensure our approaches are consistent with one another. If anything happens in the United States, the authorities there will notify us and vice versa. We are in daily contact to stay abreast of what is happening on both sides of the border and to prevent infection on either side.