I don't know how they close that "loophole". Chain of custody is the standard for good reason. It's failures like this that put incentive on testing organizations to conduct operations with perfection.
Just like when a cop fails to read Miranda rights to a suspect, it's a silly technicality but it still has to be done.
If the loophole is the FedEx date, a trusted currier had it in their possession an extra day- that seems easy to allow- after all, the same person has opportunity to tamper in the day they were allowed to have possession of the sample
As far as cops go, the bill of rights is a lot harder to alter than MLB drug policy