Wouldn't they have gone for a proper replacement, then? I mean, if he really had wanted to play with a proper striker, Napoli would have had two to three players of that kind under contract instead of only the (albeit talented) fairly young, relatively unproven and especially quite injury-prone Milik.
I think it is much more likely that Sarri has a different understanding of "proper strikers" than we have. I guess in his definition, Mertens is a proper striker because he brings everything to the table he needs. Qualities such as defending the ball with the back to the opponent's goal, keeping the CBs busy or converting high crosses are probably minor criteria. Runs, combination play, agility, finishing and dribbling as well are probably the skills he's looking for and Morata and Giroud don't fit that description while Higuain, Mertens, Milik, Insigne etc. do for at least large parts although some of them are "proper 9s" and some aren't.