That sounds like a more efficient way to do it but that’s not what I’ve seen. Saw it up close at our residence when I lived in Taiwan way back. Broken bottle glass shards mortared in on top of a brick wall.
In the 50s and 60s as a kid I saw this all over in Mexico along with burglar bars, it was embedded glass shards I saw as well, nasty looking, that and the window bars on homes were mysterious and ominous to a 1950s American boy, little did I know those people would be taking over America a few years later.