Why do cells not spread on soft substrates?
As both @MattDMo and @WYSIWYG have already so succinctly answered this question in their comments I'm just going to attempt to expand a bit on their answers.

@MattDMo and @WYSIWYG have explained that the aforementioned paper from the question has results that are "not extensible to all cell types and environments" and "Some cells need a matrix to adhere. Adherence relays proliferative signals in these cells."

I'm just going to expand a little bit on these points... I know this addressing an erroneous assumption in the question rather than the question itself but bear with me. 

Ok, so as has already been mentioned, different cell types in a multicellular eukaryote (like a mouse, or a human) only grow in certain environments and this is really important . Think about it like this, a mouse's body is made up of many different cell types all working together to create functional organs and systems and to ultimately make up the mouse. The individual cells don't know they are all coming together to make an entire animal and a cell is designed to grow and proliferate in order to make more copies of itself and spread. So why don't all cells just grow all over the place, on top of each other and spread to all parts of the body.

The reason you and the mouse aren't a formless mass of undifferentiated cells - like a big soft tumour - is because different cell types develop in such a manner that they can only grow in a restricted, structured way in a certain environment in your body. There are mechanical and chemical signals along with unique sets of growth factors that are found in different environments in the body that make it possible for certain cell types to grow in one area while not in another. It's why only blood cells can survive suspended in the blood not adhered to anything and your fibroblast cells (they make up your structured connective tissue can't, they need to adhere to a surface to grow. The different cell types only get their signal to keep growing when they are in their proper environment.  

This is why when scientists are growing various mammalian cell cultures they have to be aware of what environment and growth factors the specific cell type they are working with requires in order to grow. Some mammalian (e.g. mouse) cells need a structured scaffold to grow them in culture and some can grow in soft or liquid culture.

Source: Hallmarks of Cancer: The Next Generation http://www.cell.com/cell/abstract/S0092-8674(11)00127-9?_returnURL=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0092867411001279%3Fshowall%3Dtrue [This is a classic paper that explains this far better than I can hope to - look for the section 'Sustaining Proliferative Signaling'.]