I'll bet that they use a lot of preemergent herbicides.

I have two patches of thyme that are, oh, 1.5 ft by 2 ft? One in a raised bed, spilling over the side into the yard, and one that is next to a sidewalk. It forms a pretty tight mat and I've yet to see weeds growing up through the plant itself. Next to it? Sure. IN it? Nope! Mine is pretty much an evergreen here in zone 7. Thanks for sharing this pic - I love it!