As someone who has made six single-engine landings in twin-engine aircraft (all unplanned), every one warrants a "mayday" to start with; in fact it's SOP with my company. It gets the emergency services cranked up in case you need them. Easy enough to downgrade later. Sounds to me as though the crew did the right thing. There's a lot of Ryan-bashing goes on here, but you can't criticise a right decision.