in an absolute emergency you will transmit on the VHF frequency in use.
For example, these people would possibly have been directed as follows:
"Contact Singapore Radio on 8942".
However, they would have maintained their previous VHF frequency on their ACP. (Audio Control Panel) Most probably Singapore VHF.
On this route we usually hear Singapore VHF almost up until we contact Saigon on VHF. (Depending on your altitude)