That is true. In fact, 4E has swarm variants for most corporeal undead, if not in the MM than in the aforementioned Open Grave supplement... it's mentioned as being one of the ways of simplifying a zombie apocalypse-style battle.

(Open Grave actually has a whole section on different ways to game out a zombie horde. Sadly, one of the few glaring mistakes in that book was they forgot to include the stats for a zombie plague, though they put the errata on the website pretty quickly.)