It sounds like they didn't need (at the time) to separate arguments from options, but they did need to separate revisions from pathspecs. So they repurposed "--" as the most familiar separator.
Probably they were trying to use familiar conventions, but when they later needed to separate arguments from options, that was a closer match for what other people were using "--" for, but it was too late.
Probably they were trying to use familiar conventions, but when they later needed to separate arguments from options, that was a closer match for what other people were using "--" for, but it was too late.