They also don't differentiate by rank. Jeff said it himself, these are just numbers with no other information and you'd be prudent not to make conclusions entirely out of them.
The Mercy numbers are more than likely driven by lower ranks where she is MASSIVELY over represented. Not saying that she isn't used a lot at high ranks but a breakdown of usage by skill rating would probably show that. On top of most of the player base being sub diamond.
The problem I have with these types of responses is that they will be taken out of context and you'll see threads pop up everywhere saying things like "Blizz admits Mercy is most played/broken but does nothing". Note that the numbers he posted were in a Blizz Forum thread about Hanzo/Widow being in every QP game according to the OP.
Insert comment about grains of salt.