This would only be true if suspension of belief was actually related to how much something varies from reality or believable consequences. Truth be told, suspension of disbelieve has nothing to do with reality or logic, and everything to do with how comfortable someone is with the tropes and fundamental assumptions of a setting.There are many who would claim magic would not alter the setting much either (I’m not one of them). But there is a difference. In most media, especially fantasy, some suspension of disbelief is required. How much? As little as you can get away with. The impact of magic and monsters on settings is arguable. The impact of firearms is much less arguable. In some ways it requires a bigger suspension of disbelief than magic IF it is not well integrated into the setting.
For example, people are completely comfortable with FTL travel in sci-fi despite it being as possible as (in your words) tossing a ball into the sky and watching it gently float up to the moon. I have never seen a Sci-Fi work actually deal with the convoluted, world-changed consequences of traveling faster than the speed of light. However, because every major sci-fi story in modern memory involves starships jumping through hyperspace to reach alien worlds, most sci-fi fans have no problem with it.
There are only two reasons why many fantasy fans feel uncomfortable with guns in fantasy:
1) Most fantasy fiction, particularly older fantasy fiction, doesn't have guns. As such, adding guns brings fans out of their comfort zone.
2) Most people have grown up on Hollywood gun and explosion physics, and thus have exaggerated and twisted ideas of what guns and explosives are capable of.
As such, most people talk about adding "barrels of gunpowder" to D&D, they aren't talking about real-world barrels of gunpowder (which I don't really know the capabilities of), but instead about Hollywood barrels of gunpowder, which can magically level buildings in glorious balls of fire. I doubt that a real keg of gunpowder is actually as powerful and gamebreaking as many people have claimed it is.
If someone grew up on fantasy fiction that includes guns, even in a wildly unrealistic manner, I think that person would be perfectly comfortable with guns in their D&D. As such, I think the arguments about realistic consequences has very little to do with the actual reasons people like or dislike guns in D&D.