It's mainly vitamins b and c that are slightly lost. When veg are boiled 30% of vitamins are lost, that's why steamed is better. So i think it's a similar comparison. Iron is heat stable so not lost, but maybe not high to begin with depending on what's in the pouch.
I did some googling and some pouches are actually fine for sugar levels. I think it's the fruit ones that are especially unhealthy, similar to orange juice or apple juice etc where you get all the sugars without the fibre so not healthy but marketed like it is.
I think it's like any meal plan though, if its varied then nutrients will be fine across a whole diet with a few pouches included. So my understanding of that article is if your baby relied on them as their main source of nutrition, then it's not nutritious.