Q.Why Do You Say Christmas Cactus Can’t Be Outside Below 50 Degrees
I put my Christmas cactus outside the first day it gets about 70 degrees outside and then I don’t bring it back in until it gets down to 25 degrees. It is extremely loaded with buds by doing this and I’ve never had the buds die from it freezing at night. You just have to make sure it’s at least 55 degrees out when you bring it back in. Otherwise it’s too big of a temperature swing.

Certified GKH Gardening Expert
The article below says it can be exposed to 40 degree temps to induce buds. The cacti is native to the Brazilian rainforest where temperatures stay above freezing.
Most houseplants should be brought inside before it gets below 50 degrees F to prevent a drastic change in the temperature from outside to inside. And also to prevent any chill damage to the plant.