I totally understand why Mr. Ashe feels that way about them trying to stay through the winter, based on what we know of their gear and supplies.
Clothing/footwear - As you mentioned, it was reported that they didn't have winter clothing and boots. Obviously, a person couldn't reasonably expect to survive at that location through the winter without these items. It is possible they had plans to obtain these items, but perished before that happened.
Shelter - It was reported that they were found in a tent, with a partially constructed lean-to type shelter nearby. A lean-to shelter is a fine emergency shelter. However, a person couldn't reasonably expect to survive in one for an entire winter. It is possible they planned on a more extensive and enclosed shelter, but just didn't get that far. Even so, it would've been extremely challenging to live in a tent, shelter, or combination of the two.
The lean-to shelter is a simple design that every woodsman or survivalist should learn to assemble. If you have the right gear and a little knowledge you can set yourself up with a comfortable shelter that will keep you warm and dry through the night.
www.survivalworld.com
Food - It was reported there was only a single package of ramen noodles found in their campsite, along with some empty soup cans. It is possible that there was food stashed elsewhere that we don't know about, or that they may have been planning on re-supplying somehow. A person couldn't reasonably expect to be able to be able to source adequate food at that location through the entire winter.
Water - It was reported they had water purification equipment. Melting snow is also an option in the winter.
So we are left wondering what the heck happened. Of course, all of this analysis is based on the assumption that people were making rational decisions using sound logic. Did they have plans to go elsewhere, or improve their chances of survival at the Gold Creek site with additional food, gear, etc.? Where was their vehicle? Who was found inside/outside the tent? It would be expected that their autopsy and toxicology work might take extra time, so I don't read too much yet into how much time it has taken. However, it could also be an indication that things aren't quite as straightforward as it appeared initially.