Batter was thin, pancakes came out thin rather than fluffy

I often use milk laced with white wine vinegar in place of buttermilk. The key is letting it sit long enough to adequately sour and thicken. I heat milk in the microwave for about 30 seconds to warm it up, then stir in 1 teaspoon of vinegar per cup and let it all sit for 5 minutes. you'll see it begin to curdle, and solids begin to rise to the top. Give it one more stir before adding to the batter. The baking soda and some of the baking powder will react with the acids in your "buttermilk" to create carbon dioxide, which causes the pancakes to rise. If you let the batter sit for a couple of minutes before dropping your pancakes onto the griddle, you'll se it begin to thicken as CO2 is created. Baking powder will also react to heat to create additional CO2. You might want to check the expiration dates on your baking soda and baking powder and replace them if expired. I've let myself get burned by expired ingredients, and it's such an unpleasant surprise. Better living through chemistry.

It's also possible that the milk+lemon juice was not a good substitute (despite it's popularity as such and the recipe suggesting it). The link tests out biscuits but it gives good details on fluffiness and texture with different substitutions, and milk+acid didn't fare well: www.seriouseats.com/2017/04/how-to-substitute-buttermilk.html

From the description of the batter, sounds like you either had too little flour or too much liquid. How did you measure the flour and the liquids?