This was probably due to the fact that until some time earlier people had not actually ever heard _anyone_ speaking, these were amongst the first movies which had audio, they just had not thought of dubbing.
But they started applying normal dubbing soon after: according to wikipedia, the "self-dubbed" movies were produced between 1929 and 1931 but then stopped.
A fun side effect of this is that the new dubbers kept the odd english accent! So if you get a copy of later movies where Hardy is dubbed by Alberto Sordi (one of the greatest actors of italian cinema) you can hear him make this funny fake accent, and mispronouncing words (e.g. very famously "stupìdo" rather than "stùpido").