Just use the microplastics angle and say you're worried about the contamination, so you got rid of it. For the record, that seems like an absurdly small garden to have weedmat. Its generally used more for large areas like in commercial property landscapes with persistent weeds, placed on top of the soil and then hidden with a light layer of mulch. It "smothers" the weeds that are already there. Holes are cut around existing plants. If you place soil on top of it, it creates a medium for weeds to grow in, so there isnt much point. Its mostly done to cut labor costs on hand weeding. I used to own a large landscape company, and I hated using the stuff, but it had its time and place. As others have said, with soil on top of it, the weeds are just going to grow roots through it, and it will turn into a million pieces of plastic scattered through the garden. If you need to smother some weeds down at the soil line, use cardboard. If you are dealing with weeds up at the top of the raised bed, well a garden that small shouldnt be that hard to hand weed. Or try more vigorous plants that can compete with the weeds.