It was predictable that they’d bullshit you. There has been a little research on this and, equally predictably, the outcome was that you’d eat fewer of the problematic purines by going for soy instead of shrooms.
I’m sorry to say “moderate levels of purines”, “high in purines” and so forth are effectively bullshit as well.
Bottom line: eating the body of any organism tends to raise your uric acid. Best get most of your nutrition from stuff based on things like seeds and milk (that would include stuff like tubers, fruits and eggs for instance).
But the bodies of most land plants have very large cells so they don’t have many of the problematic purines. And relatively to these purines, they contain a lot of minerals, vitamins and the like so by eating the parts of land plants that are unlike seeds, you might get health benefits that outweigh the impact on uric acid (indeed, the beneficial effects could actually lower some people’s uric acid in spite of the purines).
Another problem with stuff like Quorn is that its nutritional value is poorly understood compared to stuff like soy. If you care to learn how to design a fact-baed vegan diet, you’ll have an easier time relying on a mix of well-known plant products than wannabe meat substitues.