No it wasn't because from the start these were called "chevaux fiscaux" and nobody was mistaken the 2 kinds of horsepower.
At the time nobody cared about the real horsepower anyway. Until the late 80's the only benchmark that people view in term of car performance capability was the top speed and at a lesser level the 0 to 100kph numbers. The first speed limits had been introduced only in 1974 and at first been presented as temporary measures.
Italy similarly had, and possibly still keeps around for backwards compatibility, the same sort of large "cavalli fiscali", used exclusively as a measure of tax brackets, not of actual power.
At the time nobody cared about the real horsepower anyway. Until the late 80's the only benchmark that people view in term of car performance capability was the top speed and at a lesser level the 0 to 100kph numbers. The first speed limits had been introduced only in 1974 and at first been presented as temporary measures.