That's not necessarily true.
You have to call and ask the lender. They will definitely require the new buyer apply in the same manner as you, and then will offer current rates, not the promotional rate that you got, but it's doe-able. Just, not necessarily convenient, smart or cheap.
Bear in mind though, if the lender is willing to do this, some still can keep you on the hook in the event of default of the new owner.
Ultimately, just have the new owner deal with their own financing and pay off your lender clean.
It's best for you and them.