I am aware of how misleading that set of answers are. I will comment on this later.
Let me do the same math you did in more depth.
13 ft = 156 in. But "choosing" 100 PPI is not the correct path.
An image 15,600 px wide, according to the common proportion of 3:2 should be 10,400 px height or 162 Megapixels in total... There is no camera to my knowledge that has that kind of sensor.
A Phase One camera has a sensor of 100 Mpx and costs around 50,000 USD, so it is not affordable to many photographers. You will have a real limitation on finding stock photography even at that "smaller" pixel count. A photographer that uses this camera works for specific clients on specific markets.
That is not how things work, and that is, unfortunately, a reasonable conclusion by reading the answers on that thread. (I will post my answer later on that thread)
Things are done the other way.
You choose a nice decent photo or let's say 24Mpx (Common nowadays and good enough resolution)
This photo has 6,000 px wide, and then you imagine more or less how big your pixels, your detail will look at the printed size.
6,000/156 = 38.4 PPI. This is a pixel of less than a millimeter.
But in the end, if this is the photo you have, the photo you like it is ok to use it.
You can resample it and sharpen it a bit to reduce even more the pixel, so you now double the resolution to 76.9 PPI (1/3 mm)
Keep in mind that this is a perception problem. You notice more the lack of resolution on an image when you have sharp edges or small detail.
This sharpness is given by contrast between to sides of an edge. So on a blurry theme, you will notice less a pixel difference than a sharp theme.
In general, any 24Mpx (well taken) image will do fine in most cases.
About the RAW or tiff files... forget that.
A well generated JPG file is good enough to be used as a source material.
Now about the "misleading part", they are talking about viewing distances... You will probably assume that as you are sitting on a desk next to the wall, that point is your viewing distance. No.
A viewing distance is related to the human field of vision, assuming you are seeing the full picture, and more or less this viewing distance is the same as the biggest side of your images, in this case, 13 ft.
In this case, the implied viewing distance of a 13 ft wide image is 13 ft away.