[Outpox]

I believe it's a typo and should read "Although it can usually work if all you have is the subtitles file [...]". This means that you would use a reference file that would allow you to synchronize your subtitles.

[jmkb]

I could imagine having a file with correctly-synced subs in the wrong language. Maybe that would be a good reference file.

(I think you're correct subbing "subtitles file" for "video file".)