Could the ulcer question just vary based on the type of ulcer? I know that h. pylori is a big culprit in people who have ulcers but are otherwise healthy, but IBD causes ulcers too (putting the "ulcerative" in "ulcerative colitis")—and since stress is a driver of IBD flares, those are certainly ulcers by way of stress. But they occur in different portions of the gut, usually.
Absolutely does, there are multiple causes of ulcers. Canker sores in your mouth are caused by either stress or vitamin deficiency and they're still "ulcers". And what H. Pylori does is a huge cause of gastric cancer, so pathogens damaging the body is a factor. I agree with the top comment of this tree that we can't see science as set in stone, but many things have multiple causes that work separately or synergistically
