Strings of unknown length were never in play. The comment was specifically about a separator in a date, which has only 2 potential conventional lengths depending on if you're using a 2 or 4 digit year.
Length can to 2 or 4, month can be 1 or 2, day can be 1 or 2. Using separators makes that easily parseable by both humans and machines, whereas using 0s is just redundant when used on fixed-width strings or potentially confusing when used on variable-width strings.
