They had no signed contracts, but with onboarding, the ramp up, and holidays, it sounds like this was explicitly not an issue for their manager.
You're right that tenure is a common way of deciding who is going, but if HR had come in and said "we're really sorry but we're letting go the most recent joiners, no reflection on you other than your start date, you'll get a good reference based on your ramp up performance", that would have gone over much better.
People don't like things that don't make sense. In my experience, between individual advice and collective bargaining, unions do make things make more sense in this way.