I would say that goose phrase is also without much meaning. It's just a phrase. Like ok, you said a phrase? So what?
Let's bring up the Iran war - you could say a few things about how it's wrong or something and I can just reply de oppresso liber.
Ok? That's not a discussion, it's a drive-by, feel good zinger.
You claim that "de oppresso liber" doesn't mean anything? Seems to me that it's meaning was pretty clear. Actually in this context I'd say there are two distinct meanings, both the "Regime change" bullshit from the start of this war and boots on the ground for a US combat unit. Seems like a poor comeback to any "War is bad" rhetoric because in my experience all such rhetoric sort of builds in a just cause assumption and that's really all you'd be going for with this phrase.
But then you say it's not a discussion, nobody said anything about it being a discussion.