I usually mix the flour and leavening ingredients together before adding them to the fat, sugar and eggs, but I'm wondering...

I imagine the reason behind mixing the soda and baking powder into the milk, and then folding it in to the batter does have something to do with determining when the leavening begins to happen. According to the recipe intro, this was a very old recipe that has been reclaimed. Old style baking powder was single sort, and began reacting like the baking soda- nearly instantaneously. So you didn't want much time between mixing, pouring and baking. Modern baking powder is generally double acting, with a second leavening action triggered by the heat of the oven. Mixing either one into milk isn't actually going to get you much of a reaction, unless your milk is sour- but it does let you have a shot at more evenly distributing the leaveners in the batter in a shorter amount of time. If you added them in with the dry ingredients, you would get the reaction started pretty quickly, thanks to the molasses. Then when you stirred in the milk, you'd be mixing and letting some of that needed gas escape, before you hit the baking pan and oven. I don't know how detrimental it would be, it would depend on how quick you were in the milk mixing and transfer. You could certainly give it a try- but just be prepared for a more dense cake if you do. My best advice would be to prepare to move quickly, once the dry ingredients start meeting the wet.