> given how shortlived they are, they never get the chance to do much evolution
There is a very rare category of cancers for which this is not true, clonally transmissible cancers[1]. They can move from individual to individual like other pathogens. One example is the devil facial tumor disease[2] that has killed around 95% of Tasmanian devils.
1. https://en.wikipedia.org/wiki/Clonally_transmissible_cancer
2. https://en.wikipedia.org/wiki/Devil_facial_tumour_disease
This is the track they are on, but given how shortlived they are, they never get the chance to do much evolution. They only have time to learn a few tricks. Simple things like turning stuff of or up/downregulating existing systems. It's a strange step to "cannot help but switch to the previous single-celled metabolism, very inefficient compared to ours".
Theory in article seems much more likely.