The rationale for "::" can be found in the PR where the Batch filetype was added: https://github.com/geany/geany/pull/160/files/79fa6c32cf6c2534424d1729283e4a...
The author stated that `REM` would actually execute the command after but just suppresing its output. This contradicts what you cited above.
I don't know what is correct; https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-xp...) pretty much says the same as your citation.
So we can probably change the default to `REM\s`.