Both 51G and 33G are severe.
There was nothing about Alonso's crash to indicate he'd be injured or need urgent medical attention. If that was in a race, it'd have been a SC.
Red flags aren't thown JUST for an injured driver.
They are also thrown when a barrier is considered compromised so that it can be repaired, preventing a (potentially) more serious accident happening at the same spot.
In a race, if the barrier is considered compromised then a red flag is thrown.