Yes, and it's effective against small-scale explosives. How is that going to help you against a truck bomb, which seems to be the weapon of choice against embassies?
edit: Granted, a brick building isn't going to do much in such cases either, but the new embassy seems to lack an exterior security wall that would help.
Which is why the updated security guidelines for embassy construction include the use of energy dampening materials, specifically the slightly tweaked recipe for Rhino Coat which turns a normal concrete block into a nearly indestructible concrete block. Amazing stuff. You can coat a block in 2mil layer of the stuff and chuck it out a 12 story window. After a few bounces, it settles intact. They're using this stuff everywhere now.
Regardless, 'what you can't see' doesn't mean much. You can't see seismic sensors, but they do their job just the way they were designed. The guy who tries to drive a bomb laden truck in the front door, a la Beirut, won't see the the hydraulically powered steel and concrete pillars that spring up to stop and kill him...until it's too late anyway. And besides, I would expect the final product of anything to vary a bit from it's conceptual layout.
Modern security measures for threats we've seen before (truck bombs, etc.) include using distance and fast response obstacles around the building to provide security. Hard to do when you're
already in the middle of one of the world's biggest cities. In this case having the building more away from the dense population of London proper makes it easier to defend against flying attacks as well. Can't have that burning wreckage falling all over Trafalgar Square, or Parliament.
This decision would have been made regardless of who occupied the White House, and it's a sound one at that.