This is a very common substitution but in some cases you definitely need more. Good recipes will state what the batter should look like so you can adjust to the right consistency (wet and runny, thick and stiff, etc). I usually add just a splash of oil alongside to increase the fat but but always, depends on the recipe.

I would actually guess they under mixed the batter. I find a good muffin needs a little bit of that gluten build up for a nice crumb and bite. Not too much, but you can be too careful mixing ingredients and you won't get the right texture in my book.