I'm as confused about the mitigation status as others. However, the paper is clearer than the website:

> That is, while the speculative JavaScript sandbox escape is still possible, an attacker becomes limited to reading their own address space and therefore their own data. Finally, at the time of writing, Apple’s patch is publicly available [57] and is implemented in Safari Technology Preview versions 173 and newer [58].

[57] is https://github.com/WebKit/WebKit/pull/10169. At the bottom of it, an engineer continues to link ongoing hardening patches for window.open() + process isolation.