It would take almost 90 pounds of Dichlor to raise your CYA from 0 to 330 (in your 16,400 gallon pool) and this would be adding about 360 ppm FC. Of course, you probably started out with some CYA before using Dichlor, but even so it does seem strange that you got so high. Of course, if you were adding copious amounts of Dichlor regularly, say at a rate to add 2 ppm FC per day, then over 6 months this would be 360 ppm FC cumulatively so one season of dichlor could indeed give you 330 ppm CYA.

Yes, unfortunately you have to drain your pool by much more than a few inches to lower your CYA. There are ways of replacing your pool water safely without lowering your water level. These involve using a large plastic sheet on top of your pool water and then draining from underneath the sheet while adding new water on top of the sheet (letting the sheet get lower in the pool). I can't find the threads that talk about this and related techniques -- perhaps someone else will.

I also agree that it would be good to test the CYA yourself, but even any CYA over 100 is going to require significant drain/refill.

Richard