It's implemented a bit differently in Dark Souls, but I'm not sure it's night and day or anything. I just played DS2:SotFS last month for the first time, so it's fresh in my mind, but there were a number of things that I had to keep a mental note of and come back to later (requiring a specific item or key). And then from time to time you'll come across a secret area that's just too difficult at your current level, and you'll have to come back to later. Despite the different implementation, there are a lot of similarities at a conceptual/design level.
Sorry, just woke up. I'll explain a little more. While both series certainly do have exploration as a large focus, they implement it in different ways. Dark Souls is much slower-paced and has a lot of one-time secrets hidden that are immediately accessible once you find them(since you have essentially all of your movement options available from the start), while Metroid contains a multitude of hidden items that require you to obtain a certain weapon/mode of movement and usually require you to make a mental note and return to it later. Now if you were to combine these two formulas, you would end up with a game that, despite its overwhelming difficulty, would still have the gall to make you traverse across this incredibly dangerous land just to get a single item you were incapable of getting earlier. Hence, the game would lose one of its major focuses.
And then there's the matter of shortcuts and save areas. Finding a new save room/bonfire is always a great feeling, same thing with shortcuts. While I definitely do a lot more backtracking for missed secrets in Metroid, I still do quite a bit of this in the Souls games; it's nowhere near as daunting once you've mastered a certain area.
In the hypothetical scenario where From would be making a Metroid game, they'd obviously have to adjust things here or there to better fit this particular series, but I've seen enough of their work now to feel confident in their ability to pull it off. And this is strictly speaking about where I feel Dark Souls and Metroid share a lot of similarities.
Gameplay-wise, sure, I wouldn't want a Metroid game that's purely a clone of the Souls games. It still has to be a platforming/shooting-type of game, first and foremost. Would I disqualify From Software on the basis that the games they've made recently play very differently from the Metroid series? No.
I know there's some skepticism in that regard, but just look at Retro Studios. Based on their track record, it was a lot more likely that Metroid Prime would end up being a disaster than one of the best games ever made. And they've seamlessly transitioned over to making excellent Donkey Kong Country games. Any great developer can adapt, and frankly I'm always intrigued to see an established studio go a bit outside their comfort zone. While my ultimate preference would still be to see a new Metroid Prime sequel, if for whatever reason the next Metroid game would be one helmed by From Software, I'd sign up for that in a heartbeat.