It's not, though. I'd imagine Martian organisms would have a much lower shared proportion of DNA than the 86% mentioned in the article, assuming they even used DNA in the first place.
Highly likely that Martian "DNA" would be different in several fundamental aspects (for starters, the triplet codons would have entirely different "dictionary" meanings in terms of amino acids, or perhaps there would be no triplets but quartets instead or even something completely different) that it wouldn't even make sense to talk in terms of percentages -- it would be like saying a rock is 50% similar to a blue whale.
Mars->Earth or Earth->Mars seeding is a popular hypothesis. We'd still likely see a bigger difference than 86% identical, though, given the billion year timeframe involved.