Is yogurt plus milk considered buttermilk?
Technically, this is not precisely buttermilk, but it's pretty close in both composition and usage.

The term "buttermilk" can actually refer to a wide range of fermented milk varieties.  Traditionally, buttermilk was produced by allowing natural bacteria present in cream to ferment some of the sugar lactose into lactic acid.  This made churning butter from the cream easier and also helped protect the cream from spoiling.  After the butter was churned and removed, the liquid that remained would be your buttermilk (today referred to as "traditional").

Nowadays, mass-produced "cultured" buttermilk is produced by taking pasteurized low-fat milk and introducing bacterial cultures to produce lactic acid in a similar fashion.  That's similar to how yogurt is produced, but yogurt is generally allowed to ferment for longer until the milk proteins set and thicken.  There are many varieties of yogurt with slightly different cultures from each other and from cultured buttermilk, and the beginning fat content of the milk can differ too.

So, technically these are distinct, but if you use milk to thin out yogurt, you're producing a beverage that (like buttermilk) contains lactic acid, producing that distinctive tangy flavor, and which (like buttermilk) is somewhat thicker than milk.  Your identified ratio is the same as other recommendations that I found while searching around, so you can definitely use this to substitute.

There are also other substitutions available.  This thread also covers similar information, in addition to being a colorful exploration of related terms.