Grignards do transmetallation?
You'd think that reaction of an alkyl grignard with a (primary) alkyl bromide/iodide gave all sorts of grignards via metal/halogen exchange.  I guess that's what your professor means by transmetallation.  You'd also get some elimination products.  Certainly that's what would happen with alkyllithiums.

That's why for carbon-carbon bond formation reactions you'd use cuprates.  Reduced basicity, increased nucleophilicity, all that.