I don't see any offer on that page.
However, even you sign up a hosting plan with a promo code, you can still add / remove domains in the same account as many as you like. But you can't transfer domains to another account since you used a promo code to sign up the account. This is to prevent people from using promo code for themselves.
If you sign up a account without a promo code, you can transfer domains among any accounts.