Care to guess how CIA did it? I think MBS knows he's a target of secret services...I doubt he sent a normal, unencrypted sms text.
Also, the KSA is basically a one-man dictatorship at this point. I'm gonna go out on a limb and say that the Saudi COMSEC guy is not giving MBS a stern talking-to every time he does impulsive 31-year-old dictator stuff like, for example, using unsecure messaging services to communicate with his goons while they're murdering a dissident overseas.
Whoever they hire, is likely to be in touch with foreign secret services before they do the job. The fact that they have a common fixer posting on forums to buy zero days is very amateurish and shows their sorry state. USA, Russia, China, UK etc all have homegrown expert with security clearances doing that stuff.
Maybe MBS is too dumb to use open source apps like Signal, who knows?
Assumptions like these are dangerous.
Note - do not turn this into a "but her emails" discussion.
I would venture to say that a great number of leaders in the west, east, developed and developing countries could not describe how a computer works. Don’t bother asking them about encryption.
He devised a plan under which staff is trained, tested, and punished for incompliance.
Once the whole of banks ~1200 people staff got certified, they set a deadline for enforcement after which anybody caught will be fired on spot.
The day after the deadline they spotted ~60 people in the bank sending plaintext, or replying to an unsigned email. And among them, one director level employee.
They had to fire them all, including a directress who went on trying to suing them for years.
Google “nokia2mon”, the saudis are definitely not masters of tradecraft.
He was very active on a US based hacking forum for years, and everyone knew he worked for KSA.
I think MBS very much expected to be found out, but likely assumed that it would not be such a public spectacle in US media with actual implementation details.
Trump played along with it as much as he could.