Perhaps, but it can be confusing if the listener interprets it as a witness making a factual statement, leading to the rhetorical question "were you lying when

Perhaps, but it can be confusing if the listener interprets it as a witness making a factual statement, leading to the rhetorical question "were you lying when you said A, or else when you said not-A?" For legal arguments, it's more like "we contend that you can't prove A, and even if you can, you can't also prove B, and both A and B must be proven for legal liability." Which most people can understand isn't inconsistent at all. That's why the legal & ethical guidelines spell it out.