When you get a clearance, it is inculcated upon you that you absolutely do not leak cleared information. If you THINK something cleared, it's best to treat it like it is.
It's possible that there is some 10D chess happening here, but I wouldn't expect details like this to be approved for apps like Signal.
Marco Rubio absolutely knows due process is a right for all persons subject to U.S. law. It's not only a right for citizens, and having taken this right away from persons, in no meaningful way can it be said it's preserves for citizens.
The federal government is at best in abeyance. And an adversary at worst.
Conversely, we know some people are not stupid. I dislike marco Rubio's politics for example, but he's a smart guy and widely considered to be competent. And as an attorney and a US Senator of long experience in intelligence matters, there's no way he's unaware of the legal implications of using a self-destructing messaging channel.