I have a hard time accepting an older player from another league as a prospect too, but I guess it all boils down to what your definition of a prospect is. A player without any MLB experience carries some level of "prospectidum" Whether they are 19, or 26 and played in Cuba.
In terms of Benintendi being a prospect it all comes down to at bats. If you're a young Milber who gets a cup of coffee in August but return to AAA the following year and generally are unproven you are still a prospect. While I'd consider Benintendi more proven than THAT he's still to some degree, small or large, unproven. I know different scouting body's have different measures of what a prospect is but by all means Benintendi is very close to not being one.