I do believe he said "căn nhà" too.
The classifier "căn" is further in the course but to reach level 25, we can redo and redo the course, so DL should consider "căn nhà này rất rộng" is an acceptable answer.
I cannot report, cannot check "my answer should..."
Cái is just the generic classifier/counting word. It can also be paired with another like in cái chiếc xe này = this car (right here).