You're 99% correct. The one thing is that it's not necessarily how many seats they have, but how many they'll sell. On a 747, they may expect 30 no-shows, so if that plane holds 300 people in Y, they'll sell 330 tickets (just making up numbers here, but the idea is pretty close). If that Y9 really means there are only about 10 seats left for sale, in this case that would mean that they have 320 people booked on a flight that holds 300, but you would theoretically have a chance still IF they are accurate in their no-show predictions and if there aren't enough other standbys to fill those seats.

Really, those numbers aren't even designed for nonrevs. It's a major piece of the puzzle when booking passengers for a flight, either by the airline or through travel agencies (or other airlines if there's an itinerary with multiple airlines). It's also used for rebooking passengers when they misconnect or what have you. In that case, you only really care about what seats can be booked for a confirmed passenger, and they don't need to worry about standby anything.