Robert Comsol wrote:
There has to be an explanation why the Klingon Empire is so powerful in the alternate timeline with the Federation on the verge of defeat, while in the "real" reality it's there but no longer such an important player (possibly because of Praxis' explosion).
As has been pointed out, Praxis happened decades before the events of this episode. So its destruction must exist in both timelines.
Perhaps in the war future, the Klingons were so pissed off that it motivated them to go all-out in their war with the Federation? They could have embarked on a massive shipbuilding campaign that didn't happen in the 'real' timeline.