Even when Mars' atmosphere was much more dense, your not talking about a large amount of either gas existing in the atmosphere at one time. A lightning discharge could cause recombination in the area it ionizes, but again were not talking about a lot of gas so your not going to get a combustion explosion.

Mars is a relatively light planet meaning that the escape velocity is small enough that hydrogen escape very quickly. Deuterium as D2 is twice as massive or as DH is 1.5 times as massive which slows average speed of those molecules making them less likely to escape. This explains whats called the isotope fractionation, the elevated ratio of Deuterium.

I'm guessing you probably got alot of this from the article.