Second quote from Elon:
"How can you establish that the was a legal marriage, the legal means you know it happened and there is evidence that it happened. I can see if there was marriage certificate or some other documents that was submitted during non immigrate visas but a sworn statement on DS 160 and in front of officer alone do not make marriage legal. Legal according to what jurisdiction? "
..........
In their letter, there's nothing saying she got married in Rwanda, in which country, or by proxy. If the marriage was valid (in the eyes of USCIS) where it happened, then she's married (I understand she did not, but how will you get evidence of that?).
Again, best of luck to you and your family.