“It’s likely they have Ruth’s divorce decree also, since those are usually public.”
Now that you mention it, Ruth getting custody of her two boys, Mark and David, should be in the Kenyan court records as well.
It’s no doubt the case that these two large newspapers, The Daily Nation and The Standard, have access to a wide range of records and fact-checking resources, particularly those related to their home city of Nairobi (the same city where David was born and died). It’s taking people a while, it appears, to grasp the full significance of having both papers affirm that David Opiya was Ruth’s son and Mark’s brother. Sooner or later it will sink in.