I understand, but a couple of cm's accounts for less than an inch. The point is, are people getting proper leg extension with seats at this height?
You get proper leg extension based on how high you set your saddle relative to the pedal at full extension, and with proper knee angle. If the top tube is lower, you have a short head tube and can't get your 'bars near saddle height, assuming that's what one wants. To get the bars near the saddle with a standard stem (such as a Cinelli 1A), the top tube comes up to as you say near the bone, while standing. Then the seatpost will be exposed about a fistful or so.
If this is combined with a laid back seat tube and a deep BB drop, such as the 8 cm you often see on DeRosa frames, the seat tube length could work out to 56 or 57 cm.