This summary is not the rule. The rule text controls.
If a court appoints someone to serve as executor because the will didn't name one (or the named person can't serve), that person has to post the same type of security, like a bond, that an administrator would have to post. This protects the estate and the people who will inherit from it, in case the executor mismanages the money or property.
However, if the deceased person's will actually names someone to be the executor, that person usually doesn't have to post any security. This is because the deceased person chose them specifically and trusted them to handle their estate. The main exceptions are if the will itself says the executor must post security, or if certain other rules (found in Articles 3154 and 3155) require it anyway.
In short, being named executor by the will itself comes with more trust and fewer requirements. Being appointed by the court without being named in the will means the court wants extra protection in place before you can act.