To be flown on an SEP Rating it is not that the AN2 is not an MPA it is that it must be an SPA.
Part-FCL Definitions: ‘Single-pilot aircraft’ means an aircraft certificated for operation by one pilot.
Guess it depends on whether the above means 'one pilot by themselves' or 'one pilot even if they need other non-pilot crew to help them'.
I don't know the answer, but I would look to see how EASA has classified this particular aircraft?