It's all in the first 3 sentences of the article.
> You might have read that Mozilla recently switched Windows builds to clang-cl... As of next nightly (as of writing, obviously), all tier-1 platforms are now built with clang with LTO enabled.
> You might have read that Mozilla recently switched Windows builds to clang-cl... As of next nightly (as of writing, obviously), all tier-1 platforms are now built with clang with LTO enabled.
When Nightly switches, that means that the version currently being developed (64), is the first version that has been switched.