I assume he didn't actually do what you think he did.
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I know how it works. It's explained in the documentation I linked to.
-c:s copy by itself does not tell ffmpeg to copy all subtitles from source to output. It only tells ffmpeg to copy all subtitles that are being mapped from input to output without converting the subtitle format (-c copy = do not convert the track format). So you first need to map all subtitles (-map 0:s?) AND then also -c:s copy to prevent subtitle format conversions.
Please take the time to carefully read the -map documentation.