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Scalar explicitly does not implement the virtualized filesystem the OP is referring to. The original Git VFS for Windows that Microsoft designed did in fact do this, but as your second link notes, Microsoft abandoned that in favor of Scalar's totally different design which explicitly was about scaling repositories without filesystem virtualization.
There's a bunch of related features they added to Git to achieve scalability without virtualization. Those are all useful and Scalar is a welcome addition. But the need for a virtual filesystem layer for large-scale repositories is still a very real one. There are also some limitations that aren't ideal; for example Git's partial clones IIRC can only be used as a "cone" applied to the original filesystem hierarchy. More generalized designs would allow mapping any arbitrary paths in the original repository to any other path in the virtual checkout. Tools like Josh can do this today with existing Git repositories[1]. That helps you get even sparser and smaller checkouts.
The Git for Windows that was referenced isn't even that big at 300GB, by the way. Game studios regularly have repositories that exist at multi-terabyte size, and they have also converged on similar virtualization solutions. For example, Destiny 2 uses a "virtual file synchronization" layer called VirtualSync[2] that reduced the working size of their checkouts by over 98%, multiple terabytes of savings per person. And in a twist of fate, VirtualSync was implemented thanks to a feature called "ProjFS" that Microsoft added to Windows... which was motivated originally by the Git VFS for Windows they abandoned!
[1] https://github.com/josh-project/josh
[2] https://www.gdcvault.com/play/1027699/Virtual-Sync-Terabytes...
You will probably like Jujutsu, which takes much inspiration from Mercurial: https://github.com/martinvonz/jj
It isn't a 1-to-1 clone, either. But tools like revsets are there, cset evolution is "built in" to the design, etc. There is no concept of phases, we might think about adding that, but there is a concept of immutable commits (so you don't overwrite public ones.)
It also has many novel features that make it stand out. We care a lot about performance and usability. Give it a shot. I think you might be pleasantly surprised.
Disclosure: I am a developer of Jujutsu. I do it in my spare time.