I am not defending JM, but I am a supporter of our constitutional rights. If we look beyond the out of context, misleading 1 in 72 billion probability, I think there are plenty of ways his counsel may mount a defense. I am no expert on DNA but know enough that JM's results do not meet the required FBI minimum match of DNA stranding which are required for a CODIS "match". According to FBI standards, DNA matching in CODIS requires that an arrestee DNA sample meet at least 10 of 13 unique chromosomal loci strand identifiers of a convicted offender. CODIS is meant for seeing if cold cases might be resolved by linking forensic evidence from unsolved crimes to the DNA of those already convicted.
A search between forensic samples is what we have here (the Fairfax DNA does not come from a convicted offender), and there is only a weak partial match of loci. This is known as a "hit" from CODIS database trawling. In some states, this sort of "evidence" is not even admissible in court. Why? It is known that two unrelated individuals can match at 9 of 13 loci and not even be of the same race, so how a 7 in 13 loci match will hold up in an attempted capital murder trial is beyond me? They have a very tenuous forensic link between JM and the Fairfax case, IMO.
How reliable is DNA in identifying suspects?
See the following link for JM's matching loci (after signature page):
Appendix 1 of JM's DNA analysis