This isn't a prescriptivist argument about language itself, it's an argument about a spelling convention (One which is arguably more accurately descriptive of the usage of the words.). Your dictionary link, by spelling "set up" as separate words when used as a verb, agrees with the author.
It's possible that language changes will result in "set up" becoming a fixed, atomic phrase, at which point writing it as one word would make sense. But since both"I'm going to set up the computer" and "I'm going to set the computer up" still sound perfectly natural to my ears as a native speaker, I don't think we've yet reached that point.
I suppose that in snake case it might be a little clearer, but in many cases it would still be good either way that someone named their method/function/variable fairly descriptively.
However something like "cleanupInstance" is much better written cleanInstance or cleanUpInstance
setup
noun
...
set up
verb (adverb, mainly transitive)
...
That said, I agree with you regarding prescriptivism vs. descriptivism.
http://dictionary.reference.com/browse/setup?s=t
I've slowly moved from prescriptivist to descriptivist, and if you want to keep your sanity, I suggest you do too, because linguistic history has shown that people use words the way they want and will keep doing so regardless of any interventions.edit: To clarify - the author says:
I think the genesis of this expression was "to set up <sthg>" -> "set up <sthg>" -> "setup_sthg". The "to set <sthg> up" form /is/ probably more correct in the prescriptivist sense, but my take is that this isn't really a mistake, it's intentional. "setTestClassUp" sucks because it spreads the logical units of the identifier all over the place. I want the verb in the beginning and the noun after.