Basically, the episode's vault storage case for the missing episode had the correct ep # but the tape itself contained some other TV show on it. When cross-checked, the tape case that was supposed to contain the other TV show was either completely empty or blank.
Unlike the lost Doctor Who episodes which were all distributed to countries on 16mm and some on 35mm all over the world. DS was only distributed to a few countries (like Mexico), and exclusively on 16mm.
Later episodes (such as the lost one which is only 30 eps from the end) were not syndicated internationally and had no kinescopes made of them.
A tape librarian may stumble across the episode on the ABC vaults some day while looking for another tape, but that's a needle in a haystack.
A few tape copies of kine-only episodes have been upgraded or found in the intervening years, so there's always hope, but when you compare it to something like Doctor Who, it's a miracle that there's only 1 episode missing.