How is lab grown meat going to give cancer to cows?

I understand that reading articles like this while lacking all the requisite knowledge to understand it can make it seem scary, but this tech is not going to create cancerous cells. The "growth factor" they're describing is a signalling molecule. It binds to certain receptors on stem cells and allows them to differentiate into a particular type of cell, in this case skeletal muscle cells. It does a number of other things besides that help the cell grow, but it is just a signal. The cell itself is doing the work. FGF is an essential component of normal cell growth.

The problem with cultivating cells in vitro is that you aren't growing a full creature, just certain cells. These cells require a complicated set of signals to grow because they historically exist only as part of a full system. Because we're only interested in certain parts of that system (skeletal muscle, adipose tissue, connective tissue), we're the ones that have to signal the cells.

This has been an issue because growth media is hard to acquire and purify, and has historically been taken from cow fetuses (because stem cell differentiation is a large part of the task of growth factors, they are present in large quantities in fetuses). Because it's difficult to extract and refine, it is incredibly expensive. Additionally, the growth media of today is not terribly specialized - it will contain lots of different types of growth factors, not just fgf.

So the ability to create fgf in a lab is amazing. Not only will it significantly lower the cost, but it will also allow for the creation of tailored growth media, which could potentially speed up the process of growing muscle cells, for example.