It, “prophylaxis” should possibly mean “before any exposure at all and as a preventative measure it is given, even before antibodies are produced by the patient.”
That is right, antibodies cannot be produced if the patient has never been exposed to the disease. So prophylaxis can be given and called what it is: prophylaxis.
Post-exposure it MAY or MAY NOT be prophylaxis depending on whether the patient’s immune system was at all triggered. If it was triggered then the “prophylaxis” is not really “prophylaxis.