Because many people use plastic Tupperware in the microwave. I've never purchased a piece of tuperware that said it would release microplastics into the food if I put it in the microwave.
I don't know what it is lately but I feel like a lot of things labeled as "myths" and "woo woo" seem to be coming out as actually harmful, forever chemicals, fluoride in drinking water are others that come to mind.
I remember there was a bit of a trend to rubbish using microwaves, as if it harmed the food or something, but it is curious that those people actually had a point, at least when the microwave was combined with plastic containers.
My opinion, it's better to always have an open mind.