The author themselves are tyring to explain to you that "Our data are strongly suggestive of arsenic replacing phosphorus". Where suggestive =!= evidence.
Plus, read the paragraph "A world of possibilities" to see why so(FTFA).
There is no evidence. There are results that are compatible with that and other scenarios. Science is not so easy as you wish it was.
If you want absolute proof without any possibility of doubt go to church. If you want a series of ever more robust hypotheses supported by growing evidence which "suggest" possibilities, then you want science.
This is the first result. It's not foolproof but it's very, very suggestive.
Plus, read the paragraph "A world of possibilities" to see why so(FTFA).
There is no evidence. There are results that are compatible with that and other scenarios. Science is not so easy as you wish it was.