The 99.9% is less impressive than you'd think, currently they're not even keeping the same program behaviour 0.1% of the time. They also mention AST in the pipeline, not CST, so I wouldn't expect source-preservation to be a direct goal.

Also, if you use a nonstandard spacing, I'd say that's on you to preserve a mechanical Source-AST mapping if you want to use any tool that mentions dataflow analysis & transforms.

As a side note, comments are much trickier than non-standard spacing if their positioning is semantic.