Right so I just skimmed the article and nowhere did they say she wasn’t a main healer.
Here’s what they DID say:
“The most important thing for getting Mercy’s staff usage correct is understanding that Mercy’s heals are kind of meh. She’s not terrible as a healer, but in a game that has Ana, Moira, and Bap, if you’re bringing her to the game in order to do lots of healing, you’re picking wrong. Don’t get me wrong, her healing is pretty good for someone who’s not a heavy healer, but if you are defaulting to healing on her you are not maximizing her value.”
So what they are ACTUALLY saying is that if you are playing Mercy to prioritize heals, you are playing her wrong since other main healers (notice how they compared to other main healers and not Lucio and Zen) heal more than she does. Her real value comes from damage boosting, and so she should be used in compositions that don’t face tank damage cough Rein/Zarya cough so that she can spend more time getting the maximum value out of damage boost.
TL;DR: You misquoted the article, it doesn’t say what you think it does. Mercy is a main healer, always has been and likely always will be.