The sentence about working from 11:00 am until 2:00 am is probably the source of confusion there, if you don't read the part about taking breaks. 11-2 * 6/week = 90.

My work week could be described the same way, but my breaks are for 2 or 3 hours each, and with no commute (which most people don't count in their work week), it's pretty comparable. If you enjoy what you do, those two late hours working uninterrupted are great.

How many hours of that would you say you are in flow? [1]

[1] http://en.wikipedia.org/wiki/Flow_(psychology)

this is similarly ridiculously high.