Now, when the first link says "fixed" sights, does that mean they cannot be swapped for better ones?
No. Fixed sights means non adjustable. Some pistols come with adjustable rear rights. That means instead of having to drift the sight in the dovetail using a sight tool, or my personal ghetto way of a tape-wrapped flathead and a vise
, there is a little screw on the rear sight. By turning the screw clockwise or counter-clockwise the point of impact is adjusted left or right. Personally I've never had the need for adjustable sights on a defensive handgun. Regardless those sights can be replaced if you want, they're not "fixed" to the metal in the slide as I think was your question.
Edit: I started out on an M&P and have recently come back to them. I have a healthy respect for them and consider them a quality weapon at a fair price, even for us civies.