They use the national UK press as their personal Twitter account by spamming us relentlessly with PR, like they did last time.
And the official Royal Reporters seem happy to tweet out news of the Harkles private commercial ventures...
On the Sussex Royal thing, it was never their 'personal' Insta. It was part of the normal social media presence of the Royal Family, funded by the RF, managed by staff of the royal households and I think technically is probably part of the Royal Archive. It never belonged to them personally, it was a work account. So it stays there as a historic record of their short and limited working life.
It's weird that Archewell Productions/Audio/Foundation don't have standard social media accounts for news and updates. It's also weird that Harry and Meghan Sussex don't have a HarkleGram account, but then again they would have to post regular pictures of their family life, home and baby. Which might be a bit tricky if they have secrets to keep.