Where does the assumption that Mercy mains don't need a good winrate come from?

I started wondering this and went through EU and NA top 500 ranks only to find out that none of the Mercy mains had any worse winrates than their peers.

Here's the data on EU top 500, I checked the profiles of every player who had Mercy in their 3 most played heroes: https://puu.sh/wuZyC/b42193d8b6.png

I didn't write anything up from NA, but it was pretty much the same. The Mercy players had just as good winrates as anyone else as long as they had enough games played. The few people who had 30 or low 40 winrate had a low amount of games played thus making the winrate pretty irrelevant.

So where does the saying come from? I haven't seen any data to back it up and the data I gathered seems to debunk it.