It's true. It wasn't mushrooms that almost killed me, but what I thought was a carrot. It was poison hemlock. You really can die out there.
It depends on the intentions of the site. If the author is advertising it as a complete guide, then I agree with you: it's incomplete as a guide. If he's advertising it as a starting point -- start with these mushrooms which have a very low risk of harm -- then it's fine in my view.
As a random aside, a crap-ton of flowers are toxic. Flowers. If somebody hadn't told me that, I'd be eating random ones I find on hikes, because I heard of edible flowers and for some dumb reason I thought they all were.