Ehh, this again.
The truth is if you play soloq as alliance you lose most of your bgs, if you play soloq as horde you lose most of your Bgs, its not favored either way.
Now that sounds stupid right, both sides cannot be at a disadvantage someone has to win? Well, the clue is in the word soloq.
If there is a BG of 10 people vs 10 people, and 5 People on the alliance side are a Premade there are 5 free spots in that BG for soloq alliance players but 10 spots for the Horde soloq side.
On whatever side you play you have a much lower chance to get into a side with a premade rather then be faced with one (since if the enemy team is a 10man premade or all soloq doesnt matter since you dont join their side). This just means that as a soloq player your chances of joining a premade are much lower compared to facing one, on either sides.
Horde soloq players and Alliance soloq players both lose, the premade ones on both sides win (most of the time).