It's really no different from cooking something on a stove: if you have no liquid at all, things are probably going to start getting too hot, sticking on the bottom, and maybe burning. As long as there's some liquid, you'll be fine. The important thing is to add enough; if you keep having to open it to add more, it won't stay hot. This is basically the minimum level: enough that it won't boil dry in an hour or so. If you're cooking something really simple like a piece of meat, using too much water just provides more water for the flavor to get diluted into.
This is precisely what the first page you linked to says: if it's not full enough, it might all boil off. The writer is just way off about quantities. I suspect his cooker's lid doesn't fit well, or he's unnecessarily cooking on high. I don't think my slow cooker would boil dry from half full in 8 hours on low, and there's no reason to put it on high if I'm cooking that long.
The recipe at the second link isn't exactly empty - a can of cranberry sauce will provide a good amount of liquid. It also might not be a great recipe. On that note, I might suggest you look for more authoritative recipes. Perhaps check out a slow cooker cookbook from your library (these tend to be pretty common) to get an idea of the kinds of things you can do. (Of course, if all you want is a plain piece of meat, I guess you don't need much.)
In general, the issue is how long the food spends at the middle temperatures, warmer than refrigeration, and not as hot as truely safe cooking temperatures, the so called "danger zone" from 40-140 F, 4-60 C.
While an over simplification, pathogens do not grow well outside of the danger zone because it is so cold that they are inactive, or so hot that they are distressed or killed.
Slow cooking, done properly, is safe because the time spent moving through the danger zone to the safety of temperatures above about 140 F / 60 C is minimized, thus minimizing the opportunity for any pathogens to grow and create toxins; then, at temperatures above the threshold (if held for several minutes), most pathogens are killed. (Do not take this to mean food is "sterilized" as spores can remain, that can become active later.)
Foods are not cooked to a single temperature to be considered safe. Instead, there is a combination of time and temperature at which they are considered safe, assuming no other problems existed. This is mere seconds at 165 F / 74 C, but perhaps half an hour (going by memory, so don't take this as gospel) at 135 F / 57 C.
Still, general audience food safety recommendations suggest temperatures of 160 F / 71 C because the 99.9% kill time at that temperature is only a few seconds--it doesn't require careful monitoring or timing, and so is a simple rule to follow.
275 F / 135 C (I see you edited the question) is definitely a temperature range for slow roasting that is safe when done properly.
135 F / 57 C is possible, but on the borderline, although it would require very precise control not usually available in an oven; this is more the realm of sous-vide cooking techniques. It also requires longer periods held at temperature, and so is not a method for casual cooks.
100 F is not safe. It is well into the temperatures at which food is being incubated so the pathogens have a cozy environment for growth.
Best Answer
In your oven, or any cooking method which has a thermostat marked with temperatures, the heat source is designed to turn on and off as needed to keep your food [somewhat] constant at close to that selected temperature.
On the stovetop, with say the burner on medium heat, the heat source just keeps putting heat into your food at a steady rate until you turn off, or adjust, the burner. However the temperature of your food doesn't just keep increasing forever, because when you cook on top of the stove in a pot, you usually have some water in there with your food. When that water gets up to its boiling temperature, it can't get any hotter. It starts sucking up heat to become steam & starts boiling away. At that temperature, the water will take any heat that your burner puts out and use it to turn into steam & leave the food. So your pot stays at that temp, about 100 degrees C. until all the water is finished converting to steam, and the contents of your pot are dry, and free to begin increasing in temperature beyond that boiling point. Pretty soon after that the food starts to scorch and burn.
So to answer your question, yes the temperature levels off at the boiling point of water, but not because your stove guarantees it, it's because the water surrounding your food guarantees it. After your water boils away, the temperature of your pot is free to increase until it matches the temperature of your burner, at least on the bottom.