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HAML pages do not support the cmd+/ "Toggle Comment"
This would need to be implemented in the HAML support, which is currently looking for a new owner (see #104815). I suppose I'll eventually take it over myself if nobody else does it, but for 6.7 I unfortunately don't have a possibility to work on this area.
Considering as an enhancement as currently HAML is not supported in the standard distro.