I said NOTHING about a "laid out fit"

KOPS is set in stone for NOTHING (road or mountain). But it's foolish to screw with a comfortable and efficient knee to crank position to get reach right. You can really hurt your knees in either discipline if you get them out of an alignment that works for your body (maybe moreso on a mountain bike where at least I throw a lot of power through my knees to the cranks in short bursts).

I ride with a "set back" seatpost on a mountain bike too. Almost all posts are setback to some extent now (excepting the traditional Thomson straight post); mine happens to be a Thomson that, while it actually isn't set back that far, looks like it is because the post itself is bent instead of just having an offset clamp. That does not mean that my knees and cranks aren't lined up in a comfortable position for me (this happens to be pretty close to KOPS for me, but that is NOT what I'm referring to). So just saying "lots of people ride with setback posts" means NOTHING; many/most are doing so with knee/crank position (as well as weighting the rear wheel) in mind (if they even think about bike fit in the first place). Lots of mountain bikes are built with the assumption that a setback post is going on them and tubing angles are adjusted accordingly; thus you are looking at a setback post as optimal to get your position correct relative to cranks. Just because the angles are different than road bikes to give a more upright position and more stability doesn't mean you should throw position relative to cranks out the window (note, I did not say "KOPS" specifically, and I won't).

I did address the issue that a longer stem would put more weight over the front which would destabilize you on a downhill (depending on degree). Some of that has to do with the accompanying saddle to bar drop and overall rider positioning as well. I still say if you can't handle the bike with a (fairly standard) 100-120mm stem that's giving you a good fit then you need some handling skills (or a downhill bike...). I have a 100, 0* on my XC bike. Fit is still much more upright than the oldschool stretched race position and I can get all the way behind the seat (ribcage behind the seat, "sitting" on the wheel) when I need to and not loose any control of the bike. Obviously it's not "long and stretched out".

I was not saying that the OP's bike was indeed too short for her or that she NEEDED a longer stem. Just that IFF it is TRUELY too short the stem is what should be adjusted, not the saddle. That does not equate to putting a stem on it so that she's stretched out like a roadie in a time trial...

If the bike is so much too short that the stem becomes so long that it's really effecting handling then the bike is too small in the first place.

But yes, I agree, you should not be in a stretched laying down position like you are on a road bike and if you are coming from a road bike then a mountain bike that is actually a good length may feel too short. A "long" stem doesn't necessarily mean you are going too long and stretched though. And I don't think getting the seat back as far as you possibly can without thinking about your knees is productive at all (in fact I think it's dangerous).

PS: I was a mountain biker long before I was a roadie, so I'm not "guilty as charged" according to that article.