Heater hoses can be put on backwards as it does not matter which way the coolant is flowing through the heater core.
The heater hoses when put on either way will work just fine to allow the heater to work and the coolant will flow just fine through the heater core.
It's best to put the heater hoses on the same way the old heater hoses came off but in reality it does not matter.
I've put heater hoses on backwards and thought it would be a problem but I've been driving my truck that way with the heater hoses on backwards for 10 years now.
The heater is still working just fine with no issues.