Properly spec'ed anchors in concrete can support a lot of weight. Likely more than that micro. You can ask for the name and model of the anchors. They'll be rated for an amount of weight. Compare that to what your micro weighs. Done.

to clarify, he didn’t drill into the concrete. just hung the microwave on the drywall itself

My microwave oven -- the second of two -- is hung as yours is. The upper cabinet supports it. No problems.