There are more horde player in general meaning there are also more undergeared players on the horde side. So it’s possible that the effect cancels out and the average ilvl of alliance and horde is the same.
Instead of relying on gut feeling, I’d recommend collecting some ilvl data from your bg games. Then compare the average ilvl between the two factions.
I did just that in 9.0.5 and was unable to show a significant difference in average ilvl between alliance/horde.
Here is the post i made back then