You can look at it two ways:
1) The strict definition of “debt free” is just that. You have no outstanding debt, be it credit cards or auto loans or property loans of any kind, real or personal.
2) In the case of an investment, like real property, if the value of the property exceeds the balance of the financing against it (so that you have equity) though it is debt you could liquidate the asset, pay off the debt, and realize profit. That would not spoil the definition of “debt free” for me.
Being debt free would mean to me not owing any unsecured debt and not owing debt on assets that don’t appreciate in value. In your case, it’s possible that the rate of appreciation of value of the asset (your home) is in excess of the interest rate you pay on the mortgage. Owning an investment that yields equity, especially when the rate of appreciation of value exceeds the interest rate charged on the financing, is good business.