1) The authors propose that this is a distinct species based on a number of physiological and genetic tests. To quote the summary of your linked paper
In summary, the phylogenetic and genetic distinctiveness and differential phenotypic properties were sufficient to categorize these three ISS strains as members of a species distinct from other recognized Methylobacterium species. Therefore, on the basis of the data presented, strains IF7SW-B2T, IIF1SW-B5, and IIF4SW-B5 represent a novel species of the genus Methylobacterium, for which the name Methylobacterium ajmalii sp. nov. is proposed.
This indicates (emphasis mine) that they think that these isolates are distinct enough from species that we have found on Earth to make the isolates a species. This is a species that we haven't found on Earth, but unsurprisingly it is closely related to species from the same genus that have been found on Earth and on the ISS
2) The authors make no statements about how the species came to be there that I could see, only that it is a novel species from the ISS, which has not been found on Earth. 3
No mechanisms for arisal are postulated, and the authors wisely did not speculate on this at all (speculation on results is frowned upon in science). One of the reasons they did not speculate on this will be that testing mechanisms whereby speciation has occurred is very difficult, especially as they would need to mimic in the lab the conditions in the ISS and show which conditions were important for the speciation. These experiments would be technically challenging and very time consuming.
It is worth noting that we don't really know the rate of species differentiation on Earth, let alone in space, partly because Earth is so covered in bacteria that proving that one evolved in the environment is difficult as it is hard to prove that you didn't just miss it last time you looked. To quote the paper I linked above:
For example, a single gram of soil can harbour up to 1010 bacterial cells and an estimated species diversity of between 4·103 to 5·104 species .
However, the ISS is one environment where it might be easier to see these sorts of events because it is relatively micro-organism free, and we have prior samples to go back and compare to. For some comparison of the numbers, I turn to the second paper you linked, which found:
Of the total bacterial and fungal isolates that grew, 133 bacterial isolates and 81 fungal isolates were identified by <PERSON> sequencing... ...When the ASVs were summarized to the genus level, 121 taxa were detected, 77 of which could be assigned to known genera
and
Overall, the number of bacteria (combination of R2A and BA growth) isolated from the ISS from all 24 samples ranged from 6.7 × 103 to 7.8 × 1010 CFU/m2
So quite high numbers of cultivatable bacteria (CFU = colony forming units, a measure of how many bacteria will grow per sampling), indeed as high as those found in a gram of soil, but very low species diversity compared to the soil.
It is still difficult to prove that this is a new species evolved strictly on the ISS, but it does seem logically likely that evolution will happen on the ISS, as it does anywhere that life is present (as far as we know). 3