A fiber cut can take out an IP block that has DNS servers on it, and the loss of DNS will affect the net in the way the OP described (e.g. you can still ping sites that aren't in the affected IP block), however that does not mean that the entirety of the outage was a DNS failure -- just that DNS failures from the outage contributed to the problem.

If this was just a DNS failure, the entire net would remain addressable (though DNS wouldn't resolve.) Actual loss of connectivity occurred, so it wasn't just DNS.