It's too late in some ways. He's already had access. The system has been compromised. Any number of bad actors, including Musk himself, could have asked used this situation to insert their own code into the system. We will never again be able to be certain that it is secure.
I imagine this looks a lot like what people in the cyber security sector do after a breach. Audit all the code, scan all the servers, monitor everything for several months. It's a ton of work and very expensive, but there are people with lots of real-world experience unhacking systems.
I can't read the article because paywall, but I assume access was blocked by some sort of court injunction. So what'll really happen is that that stuff is how actual-patriot treasury employees will occupy their time until the ruling gets appealed to some MAGA judge and Musk waltzes right back in again.
Last I saw they still have two "special employees" with "read only" access. So he still has "read only" access
But also? They already made code changes. And while I doubt they would be able to do anything meaningful, it is not hard to add another user account or an ssh tunnel to get through the "air gap". And it would not be beyond musk et al to call that "hacking".
Just to be clear, I will absolutely create new domain users or add my own ssh keys to an authorized_keys file to escalate privs or move laterally through a network while I’m “hacking”.
Also a malicious actor opening a reverse port forward tunnel with ssh allows them to punch a hole to them on the WAN side of the network when they’re dealing with NAT or firewall rules. If a system is truly airgapped then that accomplishes nothing. You’d need something plugged in to the airgapped system or airgapped network to bridge that air gap, like a usb adapter that has a SIM card in it.
Since we are talking about payment systems that interact with other banking systems, they will not be actually air gapped. By the nature and purpose of the systems in question, they must have access to the physical Internet (even if it is entirely abstracted away under layers of VPNs and encryption).
Assuming them compromised is prudent. Physical access is total access.
Why would it stop him? All he has to do is have Trump pardon him. They can do anything they want they have the guy that can hand out get out of jail free cards.