I don't think anyone disagrees that Mayday should be called, and to fail to do so makes it more work for everyone.
What I find amazing here is that Some ppruners seem to say that if the one word is left out, all bets are off on ATC responsibility. If you don't say Mayday, ATC can just ignore you, I guess you believe. I don't agree!
ATC has a responsibility to help fill in the mistakes, gaps and errors of others, just as the pilots do, too.
If ATC clears you to land on a runway that has taxi traffic on it, you are WRONG to land. If ATC vectors you to run out of gas, and you run out of gas, you are WRONG.
The world is not a pile of rule books where we play hide the weenie to decide which person is 100% wrong, while all other occupants of the system get to stand by, lilly white.
If you fail to act in an intelligent manner to make someone else's situation better, even if it is not in your rulebook, you are wrong. Doubly so when it is in your rule book.
So, the pilots were wronger than ATC in this case, but ATC was WRONG.