Your physics/math is correct, and you agree with my point but not my phrasing of it, as I chose not to go into detail but to keep it brief.At the risk of sounding pedantic (who, me?)...
This seems to be a very common mistake, or perhaps it's just a shorthand way of saying that the *percentage* pressure differential increases at shallower depths. The actual pressure differential is completely linear, irrespective of depth. It's about 1/33 ATA (atmospheres absolute, or 14.7 PSIA) per foot in salt water, and 1/34 ATA per foot in fresh water.
For example, ascending from 66 FSW to 33 FSW the pressure declines from 3 ATA to 2 ATA, a 33.3% decrease. Ascending from 33 FSW to the surface, the pressure declines from 2 ATA to 1 ATA, a 50% decrease. But in both cases, the pressure was reduced by 1 ATA.
However, the *volume* differential is not linear and it is greater at shallower depths. This is what increases the risk of lung overexpansion injury and makes bubbles grow faster for a given ascent speed when you're shallower.
When I explain to students, (with a little bit of rounding) I explain that pressure will change one-sixth on a 15 foot rise from 60 feet, but will change one-third from 15 foot to the surface.
For a new diver all the non-linear effects of shallower depth variance are what they need to have drilled into their consciousness until it is second nature - increased risk of lung expansion injury, increased difficulty of bouyancy control, and increased air time remaining.
So be careful about announcing “common mistakes” when you don’t like the shorthand description - while you are technically correct, you are emphasizing the wrong practical message - many casual readers will absorb the absolute wrong take away and continue to think in terms of the shallow depth as “only” 15 feet.