Yes but you don't dehydrate the food. And you're not eating multiple kgs of mushrooms in a single sitting, which you would need to get anywhere close to the total amount of protein in other 'protein' options

That’s a neat site, and I was hoping your answer was in there, but they don’t have data on cooked mushrooms. We’ll have to do a bit of math.

So you’re taking the nutritional data provided and then shunting the fiber and water out of the way. Why? You can’t just eat the nutritional parts of food; you have to eat the whole thing, and that limits the amount of food, and thus nutrition, you can ingest in a day.

Were you thinking about food prep? Some water weight is lost there, certainly, but it’s not everything.

Let’s add a raw steak into the mix, and then we can instead look at how much water weight is actually lost when you prep these things to eat, by estimating it from data elsewhere.

The beans are ready to eat. They’re drained and rinsed. You don’t remove that water weight. So that’s 7 grams of protein per 100 gram serving.

The steak will lose about 25% of its weight when cooked, per multiple sources I found during a search. That means we need about 133 grams of raw beef to achieve 100 grams of cooked beef. So we can multiply its 21 grams of protein by 1.33, and we get about 29 grams of protein in a 100 gram serving. Their grilled steak averages around the same amount, so we’re on track so far.

Why is that discrepancy so great? I thought beans were supposed to be a great replacement for meat?

That comparison was done between beef and dry beans (note the 24 grams of protein, about the same as the beef). 100 grams of dry beans becomes about 370 grams of prepared beans. So in a 100 gram serving of beans you can actually eat, you get just over a quarter of that 24 grams protein: our ~7 grams from earlier. You also lose some water soluble protein when you rinse and drain them. They’re not the magic protein replacement people think they are.

Mushrooms are even worse. Per America’s Test Kitchen (and we’re gonna have to take these numbers at face value because I can’t find anything else), shiitake mushrooms lose about 14% of their weight in water when cooked, and cremini (think portobello, they’re just different stages of development) mushrooms lose about 60%. Thankfully the USDA’s site also has nutritional data listed for these two types of mushrooms: “minimally processed” shiitake and cremini mushrooms contain 2.4 and 3.1 grams of protein, respectively, per 100 gram serving. But those aren’t meal ready. To do that, we’ll cook the mushrooms, and they’ll shrink to 86 gram and 40 gram servings. So let’s start with enough raw mushrooms—119 grams of shiitake (or 119% of the original serving) and 250 grams (250%) of cremini. Multiply our proteins by 1.19 and 2.5 and we get a plausible range of between 3 and 8 grams of protein per serving. So some are comparable to beans in their protein content! And some contain half, or less, of an already low amount when compared to the protein found in meat.

This quick comparison on Wolfram Alpha shows a similar story, with a less optimistic look at mushrooms’ possible protein content. Screenshot:

Now, the fact that you’re taking in so much more water when you eat 100 grams of beans or mushrooms than you are when you eat meat means you can eat more of them, and drink less fluids, but only to a point. And you’re certainly not getting 8 times more mushrooms than beef from a restaurant when they do a protein substitution. Getting enough protein in a vegetarian or vegan diet can be hard work. And restaurants are not making it easier by misleading people who may not know any better—I’m certain it’s careless, not malicious, but it is happening either way.