The article dissects what's on Hesgeth's desk behind him. First, there's a personal computer there connected to the open web. The article says, "He wanted this computer to use the messaging app Signal, which is the preferred method of communication among Trump's government officials."
Right next to that computer is a "Cisco IP Phone 8851 with a 14-key expansion module." That phone "connects the President, the National Security Council, Cabinet members, the Joint Chiefs of Staff, intelligence agency watch centers, and others." IOW everybody ostensibly on that Signal chat except the editor of The Atlantic.
So now I'm questioning what "functional" means in this context. Does it mean "A thing I can type into while I'm at my desk and can continue to use while I'm taking a dump as long as I poop in public wifi", or does it mean "a thing that brings all top staff together to truly "have op-sec"?
Reading further it looks like he also had access to "SecDef Cables", which provides " interoperable, certified and accredited, multi-security level voice, video, and data services."
So there are functional alternatives, especially considering the functions I personally thought our government was looking for. Maybe they prioritized a safe space for Waltz to use his favorite emojis instead?