A curious claim over doping positive

Stefan Schumacher, the former Gerolsteiner rider, has always claimed innocence over his positive test for CERA (and EPO derivative) during the 2008 Tour de France and the Beijing Olympics.  That comes as no surprise - few athletes admit to cheating, even when presented with pretty unequivocal evidence.

The announcements that samples taken during the 2008 Tour would be re-tested, and secondly that those 17 tests proved negative have apparently given new life to Schumacher's claims of innocence (Schumacher calls for withdrawal of positive results).

This strikes me as somewhat strange:  Scumacher tested positive for CERA using the then current version of the CERA test (as I recall, CERA had been touted amongst the dopers as an undetectable form of EPO - incorrectly as it turned out): now 17 riders have been retested and found not to have been taking CERA.  And this invalidates Schumacher's positive test how?

Interestingly, Schumacher's positive test results at the Tour and at the Olympics were conducted independently by two agencies.  Isn't this a bit of an open and shut case?