I think you make an excellent point.
Both the M9 and S5 cameras are f/2.2, pixels are both 1.2 microns. The M9 gets more pixels with a slightly larger sensor and slightly larger aperture (lens diameter) but at these geometries, that's splitting hairs.
Assuming his non-pro skills are the same as yours, you've confirmed that the M9 low light pictures look about the same as the S5.
I'm not surprised and it sets a reasonable expectation - rather than the greatest dog of all time, the M9 is going to perform similarly to the S5 in low light because you can't escape physics (unless you're
@Hadron on vacation, not sure lol).
My son-in-law has an S5, I can confirm that it's not great in low light, especially with the default settings. (Changing them results in a noticeable improvement, true for most every camera of every type I've used.)
The S6 is an f/1.9 setup - I would expect sight unseen for it to do better in low light, all else being basically equal because that's what the f-stop number is there to tell you.
http://en.wikipedia.org/wiki/F-number
So, worst case, the M9 will be like an S5 in low light.
Under normal lighting or with an experienced hand at the controls, image quality goes up from there.
That's all based on the earlier software.
So if the questions are - how does it perform in its own right, what can I expect, and is it really the worst camera of all time - pretty sure that those questions are answered.
If that's you and you're good with the expectation, then the M9 may be right for you.
If the question is - how do I get the best low light performance, period - then the answer is the same as it always will be. Get the phone with the lowest f-stop and largest sensor with the best lens, from a decent manufacturer so the software doesn't suck entirely.
So on the other hand, if the second case is more you, you probably want to consider the S9 (or similar if it occurs this year) over the M9. (edit - If you want the largest sensor and best lens, get a Panasonic Lumix CM1.)
If your concern is for low light selfies, then the M9 is very probably going to win you over because it's an f/2 Ultrapixel shooter.