Above ground and indoors, sure. I presumed the author meant typical CAT cabling. There is ethernet cable that is rated for outdoor and burial,and that should be suitable (but probably more expensive than sprinkler wire anyway).
I guess it depends on the application. Of course, purely from a point of view of electricity traversal, wire is wire is wire… if it can handle the current, then it really doesn’t matter if, for example, you’re just temporarily hooking it up for testing or if you’re housing the valves indoors. But if you’re doing a “normal” sprinkler install, then why go through the trouble of trenching and backfilling just to have to do it again because rocks have rubbed or insects/animals have eaten through the sheathing? Corrosion, thermal stability, flexibility of the core within the insulation… it all plays a role when doing an outdoor and (especially) subterranean installation.
Just for some reference: http://en.wikipedia.org/wiki/Direct-buried_cable