What a crock.
He is not immune to presecution, although he IS immune to arrest while commuting to/while "in Session" - these laws were designed to protect the legislative process from executive abuse - not to protect the representative from the consequences of his actions.
I think this interpretation is a good one. The law prevents the use of physical arrest, a power which could easily be abused; for example, a legislator could not be physically arrested and detained to prevent him/her from a vote. Since an arrest is not subject to any particular judicial oversight at the time, it could be used as a handy mechanism to keep away legislators. To say that they have blanket immunity from prosecution is absurd. Not to mention that the guy is using a pretty lax definition of official business. I hope they throw the book at him, and hard!