The real question isn't whether they exist - they do - but whether that Australian story tells you anything about actual availability through normal funeral channels. Mycelium coffins are manufactured and you can buy them, but there's a gap between "this product is real" and "your local funeral home will offer it as a standard option." That specific case might've been someone going out of their way to arrange something unusual rather than picking from a menu at a funeral home.
The coffins themselves are legit. Companies have been making them for a few years now - they're grown from mushroom mycelium and designed to break down quickly in soil, which is the whole appeal. The environmental argument is solid: faster decomposition, no chemical treatment, less resource-intensive than traditional timber. But availability's the sticking point. In most places you'll need to source one independently and then coordinate with a funeral home willing to work with it, rather than just calling up and saying "I'll take option C: the mushroom one." Australia might be moving faster than some countries on this, but I'd guess that news story highlights an exception rather than a shift toward standard practice. Worth asking your funeral home directly if it matters to you, but don't expect it on the form yet.