He read back a clearance and wasn't challenged when he got it wrong. Who said he wasn't happy with it?
For descent into MRU the enroute controller CPDLCs to contact radar when ready for descent. At this point you are on the limit of VHF range. Some transmissions are good, some carrier wave only. So although you can hear all the transmissions on that YouTube clip, that does not mean the crew heard all of them. Who knows if the crew took action to avoid the conflict without asking them? The approaching traffic would have been displayed to a max of about 80 to 100nm away only when it was within 9000 feet of their ALT. Nothing abnormal about that until they leveled and the traffic kept climbing as it was cleared to.
Many holes in the safety net here. Lucky training and TCAS prevented something bad from happening.
No crew intentionally goes out of their way to cause an incident like this. But everyone can have a bad day.
Good learning points for everybody.