PersonFromPorlock:
It has been clearly explained by the Russian pilots that they did not listen to 121.5 MHz because it is an emergency channel for them and there was no state of emergency, and they were busy with the operation.
The point about guard channel is that it's normally on in RX mode, regardless of the radio's 'working' frequency. There is no need to switch to it. If they had guard channel off (possible, but not wise) then they have some small responsibility for the situation.
I'd still like confirmation that the Russian military guard frequency is now 121.5/243.0.