In that topic you linked to, forum user MING goes on to say this:
So it could be that unlike (say) Ana, Mercy doesn’t get affected that badly. I can see this lingering heal effect making up for lost healing output, at the cost of needing to switch targets instead of pocketing one single teammate to put out maximum HPS. It wouldn’t make sense to nerf healing equally across the board when not all healing is high; why would they nerf Lucio’s healing, for example.