"Expert" is a relative term. He may not know all the intricate details. But having to live in the system he has first-hand knowledge, and not being part of the system (apparently) he has no personal reason to hide its dirty secrets. Also, his experience is likely to be typical for other people like him. This, I would say, makes him a great source. Then if an "expert" wants to perform a scientific study or in-depth journalistic investigation or some other thing "experts" are supposed to do, the said expert would need to contact him and other like him, for knowledge. Which he, thankfully, has freely shared with us here.

Corruption in Italy is hardwired in the behavior of people and their approach to life. It's enough to live in Italy for a long enough time to understand how people reason and why corruption is so widespread.

Obviously, being Italian doesn't make me an expert in corruption from a legal point of view, but I am pretty positive to know a bit about Italian mentality and facts confirm that corruption is as common in the North as it is in the South (I can cite my sources, if necessary.)

Chances of an Italian being an expert on Italian corruption far outweigh the chances of other nationalities being experts on that subject.

So even if it isn't ironclad evidence it definitely moves the posterior probability towards 'expert'.