As much as I enjoy TUC, it doesn't do a good job of establishing where Praxis is relative to Kronos. We know that it's close enough that it has serious environmental implications. However, that would imply that the borders of the Klingon Empire are such that Kronos and Praxis really straddle the edge, because that's really the only way the Excelsior could be close enough to get hit with the shockwave without crossing into Klingon space.
I guess that's for another topic, though.
It was a sub-space shockwave, so really it could be anywhere.
My thoughts are that it is probably suppose to be Praxis we are seeing. Nero's incursion, on 75,000 km from the Klingon border, caused them to begin an unprecedented military build-up and caused the Praxis disaster earlier in this timeline.