What a mistake to make! Wonder if he's taken any other of his wife's pills by accident?! ;) Seriously though - it is a bit of a minefield out there, but this shows that the testing does actually work.
Being cynical, I would say this shows that the testing works when a player doesn't realise that they have ingested a particular substance and therefore takes no steps to make it undetectable. Also possible that this is an easier-to-detect substance than a lot of what's out there (though I've no idea).
Not trying to cast aspersions on Johnson or anyone else in particular at all - his story in this case seems solid - but I wouldn't take one positive test for a random substance that the player took by accident as proof that there isn't a wider PEDs problem out there. For a start, the amount of testing in premiership rugby is fairly pitiful (I think it works out as less than one test per player per season, whereas a cyclist who leads the tour de france from day 1 would get tested 21 times in 3 weeks.).