That's what I thought to. But they actually made the under-the-dumpster move believable.
The walkers crawled in after Glenn. He stabbed them in the head with his knife and their bodies acted as a barrier. He was sealed in by walkers. The rest of them eventually lost interest (having feasted on Nicholas) and wandered away.
I think the “wussification” of Glenn was irritating and a harbinger of Morgan. Really. Glenn took this Nicholas under his widdle wing, after watching how he got Noah killed....eaten alive???? ick. Audiences like it better when the actors are competent, forceful. Glenn in the beginning, not Glenn now