I'm not saying this is why, but one possibility, which I vaguely remember reading here once upon a time, could be, that Aegean has intentionally left the seat selection for this flight (or other similar flights) unavailable, because they want to be flexible with their equipment, ergo leave themselves with some space to change aircraft on short notice, which could have a different seating configuration and/or a different total number of seats available.
But of course it could be due to a number of other operational reasons.
Glad to hear the hotline was able to help and I have no idea why the social media team made the statements that they made (I've also never used those reps since I don't use those social media anyway, but at least 90% of the time the phone agents have more options for you, so that's probably where you should generally start )