Could it be instead that civilization caused a general lowering of testosterone..?
It *doesn't* work that way. Natural selection causes traits beneficial to survival/reproduction to become dominant.
"Civilzation" cannot "cause" anything.
The conclusion reached by the researches is also nothing but a romanticized opinion, as we don't see lower levels of aggression in current humans.
The most likely (and brutal) cause-effect relationship is that tool makers, who were smarter and at least as aggressive despite their testosterone leves, dispatched the less intelligent high testosterone rivals and/or outbred them.