I wouldn't be surprised if it were partially causal.
But I think it's pretty clear that any high calorie diet - even one not associated with weight gain - increases the risk of cancer. Energy dense foods aren't good for us. When used for the express purpose of gaining weight (i.e. going on a bulk) then yeah, it does increase the risk. Particularly given the over-prevalence of guys who bulk and never properly cut.
Even if you do cut that period of eating energy dense food and putting on weight and cutting is probably posing an increased risk of some diseases. So does drinking, smoking etc. We all make our own choices. But it is important to have that information out there.
I really don't see why they would lump creatine and protein powder together though. Completely different things.
This is what I referred to in a previous discussion on how bulking is unhealthy. Think it was a week or two ago.
There are numerous studies showing that fasting and caloric restriction increase lifespan and improve health, but that is not optimal for getting big and strong.
Additionally, many of the dietary choices that create a better "anabolic environment" (milk products, high GI carbs, red meat, saturated fats) negatively affect your health. For instance, going vegetarian is generally better for your health but not gaining muscle. As an example, the national dietary guidelines for meat consumption in Sweden is 200 grams / 0.5 lbs per week. Many lifters surpass that every day.