so if i'm understanding your reply correctly, a lab technician may not necessarily examine a sample and think "oh, this appears to be diluted, let me send it to the lab for further testing". rather, it's a matter of whether or not it gets sent to the lab in the first place, unless the eCup technology/tester is sophisticated enough to successfully detect creatinine or S/G levels.
Cup-type assays are available with additional validity panels for creatinine & S/G.

Did the technician perform the cup test in your view? If so, you would notice the type of cup test being used.

the conclusion i'm drawing here is that the 20-ounce gatorade, unless i am extremely unlucky, will not be enough to trigger a negative-dilute result. does that sound about right?
That would depend on how much fluids youve consumed prior to consuming the gatorade and what was your creatinine and S/G levels were before consuming the gatorade - and you will not know this unless youve ran your own validity test on a urine sample before (and after) consuming the Gatorade.