You disagree over opinions. Should Signal be an appropriate system for discussing classified data? I'd say no, you might say yes, we disagree and debate.
Legally, is Signal an appropriate system for discussing classified data? No. Unless you believe in alternative facts, there is no point to disagree on, it's just a fact that it is not legally an appropriate system for what they did.
And then swiftymon lied and used "evidence" to bolster their lie that didn't even agree with their lie.