Why would you say that? Large networks have a ton of routers in them, and even a lot of switches provide routing functionality.
I don't know the details of the environment, but even in smaller systems I've worked on there is a fair bit of hardware separation between various network segments. Complete failure on one part would not affect the others.
For that matter, even a slight corruption in some ARP caches, or stale internal tables, etc., could cause the problems they had... it's not just a complete failure that could cause problems.
And "routing" is such a generic term, when it could really be any number of feature sets that failed; load balancing, source routing configs, etc.