The fact is, it is a lot easier to reach NDLs on air than on nitrox. The average air diver in Cozumel will likely be approaching NDLs more more closely that the average nitrox diver, especially if diving in a group of air divers where the profile will be tailored to air limits. Therefore, the nitrox divers will be absorbing less nitrogen.
Also, as you say, the 10-15' stop provides that more efficient off-gassing. Don't you believe that the offgassing is even more efficient and effective on nitrox than on air? 32% might not be the best deco gas, but it sure beats offgassing on 21%. For this reason, diving nitrox on nitrox tables is safer than diving air on air tables, provided one does a "safety" stop and reaps the benefit of offgassing on a higher ppO2.
I agree with you in theory - but my point is that Tracy's information is misleading and inaccurate in the way she stated and appears to understand it (I could be wrong of course - just basing it on how I interpreted her post). On nitrox, if exceeding the air limits, you are still absorbing more nitrogen even if not reaching the nitrox NDL limits - but reaching nitrox NDL limits is certainly no different than air on air limits. The point is that while it can decrease nitrogen loading, it s not necessarily true - and often NOT true depending on the diver and the profile. In her example of using 28% on a deeper dive to extend the bottom time, a diver with good air management could in fact reach and exceed the nitrox NDL limits.