Keeping it simple`
Even though it may not be, but paring back the extraneous issues.
1. Pilots have preflight briefing of route
2. Route is changed. Pilots unaware.
3. On the day pilots think they are flying into an area where they know they can descend to xxx ft.
4. They look out the window. They are clear of cloud, in VMC.
5. There is cloud above that stretches to the horizon. It causes whiteout (NOT IMC), obscuring any surface definition that would otherwise alert them to their being in the wrong spot.
6. The accident.
Yes it was a CFIT.
They could not (through inadequate training perhaps) recognise the dangers and were lulled into a sense of security by being VMC and where they thought they were.