It's true they don't update all their computers at once, but I find it unlikely that they will have inconsistent update cycles for two laptops that are essentially the same apart from screen size. The iMac hasn't been updated, but it's alone in its product category right now.
There isn't really a direct parallel for this right now, because usually they don't release different screen sizes years apart. This is a new product, but it's not a new product line, it's just an additional size option for a product line that has had only one screen size option for a while now. It seems like there would be more motive to get these two laptops on the same page than to, say, get the iMac on the same page as the MBA.