It has always been related to departure. The law also applies to passengers on ships. Long ago I gave the example of someone on a cruise liner from New York where it would have been impossible to have done a PCR test within 3 days of arrival. Whilst a rapid test would be possible it would still be very difficult to stop someone disembarking a ship because of a positive result from an onboard test - and hard to explain to the boarding passengers. Maybe by accident and perhaps by design this part of the law actually works in a sensible way.