It has some page where it almost goes out of its way to say that it doesn’t bypass security measures like Nitter and XCancel supposedly did. So Elon might not have even a fig leaf of a legal argument to take it down like he did with the other websites.
It’s also possible that Twitterviewer is just flat out disobeying cease and desist letters from Musk, which nitter and xcancel weren’t willing to risk.
In any case, I find it unlikely that Musk hasn’t heard of Twitterviewer.