When our undermount sink was installed, they setup a temporary support to allow the silicone to setup....it was removed about 48 hours afterwards....I can't imagine that they intend to leave it like that....but ask them!

I have wood support under my sink that can be removed to do plumbing work.

Unless your cabinets have a full plywood subtom, 2cm granite REQUIRES a plywood support underneath. And no, that isn't what I'd expect to see from a professional sink installation. Not at all. I'd want sink clips or a Sink Setter. If that's a brace, it's crappily done as well. They screwed into the cabinet sides? A temp brace is usually done with a clamp through the drain hole and a 2x4 across the top of the sink. Once things set, you take the clamp off, and it's perfect.