Current scientific theories for evolution of Cucurbita
You're right that modern species would have a hard time spreading group seeds, though you're not quite right about the reasons.  Cucurbita rinds are extremely bitter, so that modern mammals are reluctant to (or unable to) process the rinds to access the seeds.  

According to a recent article (Gourds and squashes (Cucurbita spp.) adapted to megafaunal extinction and ecological anachronism through domestication), the explanation is that wild Cucurbita first co-evolved with North American megafauna, like mammoths and mastodons, who could tolerate the toxic/bitter rind. (The authors also suggest that megafauna also produced suitable habitats for their growth.)  

Following the extinction of these megafauna, humans picked up the slack, with most wild Cucurbita going extinct and only domesticated versions remaining.


  Collectively, these results point to wild-type declines coinciding with widespread domestication. ...megafauna consumed Cucurbita fruits and dispersed their seeds; wild Cucurbita were likely left without mutualistic dispersal partners in the Holocene because they are unpalatable to smaller surviving mammals with more bitter taste receptor genes. ... Thus, anthropogenic landscapes provided favorable growth habitats and willing dispersal partners in the wake of ecological upheaval.