You're not understanding the difference between a mesh and a solid. Fusion cannot machine meshes... so they are not telling you because it's impossible.
You can convert your mesh to a solid that fusion can understands, but then you cant unwrap a solid... it's a solid. You can unwrap a mesh because it's a shell... it's simply a surface with no volume. If you unwrapped your mesh and then converted it to a solid so fusion could machine it, then it would be a solid again. So, there is no way to get around this issue.
In the same way, Vectric can unwrap a mesh but it cannot machine a solid. Thus, Vectric can wrap on the 4th axis, but it cannot index on the 4th axis.
These limitations are because of the models used and the associated advantages/disadvantages of both.