It doesn't rain Solllecito DNA, there are only a few traces of Sollecito DNA, which is consistent with second hand transfer.
If Sollecito DNA was going to be found through transfer, it would likely be to objects that AK would have routinely touched, and those same objects would have for the most part also been routinely touched by MK. If she then subsequently put her bra on she would have touched the clasp with 100% certainty (that is how bras work) and transferred the DNA there.
So, there is a reasonable explanation as to how his DNA could have got there, you do not have to invoke murder to explain it.
If they were going to make arguments about Sollecito being there they would need a lot more than that. For a start, they should be able to find at least a few fingerprints, something, but there is virtually nothing.