So we have to surmise the sample was so small they couldn't extract all 13 markers. Yet the AV clawed away and only came up with a minute sample?
Finally, the girl was asked to leave, they slipped money under the door asking her to leave according to Kim who was/wasn't in the bathroom with her? Is it possible the guys "helped" her out the door? They did lock her out.
Bottom line, none of this is evidence of rape.
I think "small" is not the exactly correct description because as Jeze points out even one cell has it all. So I think "compromised" or "damaged" is better.
The point is that there was a vanishingly small amount of DNA on the nail that could have gotten there from any number of mechanisms.
I agree with your bottom line.