I don't believe that's true. Using a Credit Card funds can still be held and either released or taken at a later date. This usually happens with you rent a car and they need a security deposit.

What I don't understand tho is /why/ a merchant will often accept a credit card but not a debit credit card. While there's differences in fruad protection and refunding, the main difference is debit credit card draws on funds you have while credit card supplies a line of credit to re-pay at a later date.