If I understand correctly the physical sign points to the sea direction i.e. in opposite direction of how OSM shows it.
I checked google street view and the Senso Unico arriving from Bixio at the berardinucci points at one way to the left. All cars are parked in there in the direction of Bovio and at the end Berardinucci there's no senza unica sign and there is a No-Entry sign indicating the one way is in opposite direction. All seems well now with the OSM routing. For completeness will put a No-Entry sign on the OSM map where Berardinucci enters Bovio. Concur and close the comment as resolved.