I mean no offense but this answer appears to be almost entirely speculation and based on a cursory understanding of the processes at best.

> Fruit rot isn't really a sickness, though, is it?

Well, as with anything that can rot, it depends an awful lot on when exactly it does so, doesn't it? If the pumpkin is off the vine then rotting is not a sickness. If the pumpkin is still on the vine though, which is the situation OP specified, then something atypical is going on, perhaps a fungal or bacterial infection such as by Fusarium. We cannot speculate on what kind of rot it was with so little information but we absolutely cannot come to the conclusion that there were no pathogens involved.

> Rot is a part of that process not an infection.

I literally cannot think of any plant that rots a fleshy fruit as its primary means of dispersal. Some that can survive rot but that is not the same thing as relying on it. If someone has an example please let me know, with an article describing the species, because it is fascinating.

> If the pumpkin doesnt rot, it's just sealing the seeds into a container, which is counterproductively preventing them from germinating

You are correct that sealing seeds into a container is counterproductive but your conclusions from this are not. Producing a large and fleshy fruit like a pumpkin only to depend on it decaying would be very inefficient. It's a large resource investment and puts the seeds at risk as the fruit decays. You might ask "Well wouldn't the rotting fruit provide nutrients for the seeds?" and while yes it might, it would be more efficient just to put those resources into making the seeds themselves larger or perhaps producing many more seeds.

There are indeed fruit that are not "meant" to be eaten and this is actually rather common, but these are almost always dry rather than fleshy. They might contain only a single seed and be adapted to fly, such as the samara used by maples or the cypsela of the dandelion, or perhaps they have a single seed and float instead like you see in coconuts. Maybe they contain many seeds and the fruit cracks open at maturity, like is common among orchids and milkweeds. Or, perhaps, as is seen in jewelweed and a few members of the pumpkin family, the fruit violently bursts apart at maturity and the seeds go flying. I'd need to get a lot of articles for this section so I won't do it now but I am happy to provide article about specific examples on request.

> You could argue that some fruits aren't evolved to rot

This is every fruit I can think of but I repeat myself.

> thus propagating farther and ending up deposited in manure

This is very common for fleshy fruit, yes. Animals can move much further than plants so it's a good way to ensure good dispersal.

> I think avacodos fall into this category with some extinct megafauna being the intended propagator.

This is actually true. It is believed that the giant ground sloth was the primary dispersal method for avocados prior to its extinction.

> But I dont think there's a candidate animal that swallowed pumpkins whole. Plus pumpkin seeds don't appear to be tough like an avocado pit to survive the gut.

I cannot for the life of me imagine why you think seed dispersal requires the fruit to be swallowed whole. It most definitely does not. Some bird dispersed fruit may be swallowed whole but overall there's a very high chance that a fruit is getting chewed up by whatever animal disperses its seeds. Do seeds die in this process? Yes, of course. Is this a problem for the plant species? No, because some of the seeds make it through intact and can grow on the other side.

As for the candidate animal, there is some evidence that mastodons were responsible and, like the ground sloth, humans took over as the primary means of dispersal following their extinction. Animals such as bears will certainly eat them now but I cannot find evidence that they also ate the more bitter wild versions.