I think this is underselling it tbh. It got strong critical acclaim and good first week sales - 3 million copies sold in a week is a position most devs would kill to be in. That's not even accounting for the fact that it's an exclusive to a platform with only ~40m units sold, or that it is a $70 game, both of which would have had some effect on sales (how much of an effect is unclear and likely not something we'll ever know, but certainly there was some effect).
The issue is one of perception. Square almost certainly hoped it would sell better than what it did, and as you and others have discussed, it certainly isn't The Main RPG Series anymore, and the sales reflect that. But I think saying it's somewhere in the middle of a breakout success and a flop kind of implies that it's near to halfway point between the two extremes, when it's really nowhere near a flop.