Can a Dremel Circle Cutter be used on 20mm thick wood?
As others have said, this will work, IF you have a sufficiently long bit, that will extend at last 2cm below the base of the jig. The problem is, most such bits for a Dremel tool are not that long. You may be able to use a Rotozip bit, which are longer than the standard Dremel bits.

Next problem is, even if you do find a sufficiently long bit, you need to deal with flexing of the bit, or breakage. These bits are relatively thin, so they can break off. Therefore, you will need to go slowly. Don't push it faster than it wants to cut. If you feel too much resistance, don't push as hard.