I am pretty sure no conformance checks, as some others have said, but I also have seen huge backups at YYZ transborder CBP in the past four or five months, since even before COVID19. Particularly relevant, I have seen people stuck in CBP still 20 minutes before their flight departure who were not permitted to cut the line (they were pleading with staff), and seemingly they missed their flight.
So I am guessing UA has had some passengers miss flights recently and adopted more scary language as a result.