A current hypothesis to explain this is collisional removal of the mantle from impact during the closing stages of planetary accretion. In the early solar system the orbit of Mercury inward would have been a high energy environment. Comets or other objects in eccentric orbits around the Sun accelerate as they approach. Impacts on the mercurian surface, then and now, occur with great energy. A cometary object impacting Mercury, prior to or during its eventual complete differentiation from heat might have blasted pieces of both the outer chondritic layer and core into eventual parking or holding orbits within the asteroid belt, that later crossed Earth's path.
Breccias are from the surface of things.
Current studies seem to present difficulties for other than an interesting connection between enstatites, the inner solar system, and Mercury. I don't know. But absence of proof is not proof of absence. We learned that from the Albion iron. The author recognizes the controversial nature of possible Mercurian meteorites and recognizes the arguments for and against a connection between E chondrites, the inner solar system, and Mercury.