Wikipedia claims (without reference) that
Metastatic cancer cells often express a high density of sialic
acid-rich glycoproteins. This overexpression of sialic acid on
surfaces creates a negative charge on cell membranes. This creates
repulsion between cells (cell opposition) and helps these
late-stage cancer cells enter the blood stream.
so I assume this is a summary of the "50 or 60 years of cancer research" that you mentioned. I don't doubt you, I've just never read up on this particular aspect of cancer. Be that as it may, there is a difference between a cell's expressing high levels of sialoglycoproteins overall, and inducing high sialylation of one protein in particular, in this case EGFR.
As you may be aware, EGFR dimerization induced by multiple methods, including EGF binding, overexpression, mutation, chemical or antibody crosslinking, etc. activates tyrosine kinase domains in its intracellular region to auto-phosphorylate specific Tyr residues in its tail, which then recruit adapter and scaffolding proteins which support downstream signal transduction.
The paper you read shows how increasing sialylation of EGFR reduces its trans-affinity for itself and inhibits dimerization, and hence the downstream signaling cascade. This is a good thing in certain types of cancer which are driven by aberrant EGFR signaling, whether through overexpression of EGF and/or EGFR, mutations in the kinase domains, mutations in the dimerization domains, or otherwise.
This paper is also a great example of an apparent bad thing turning out to be good, when properly targeted and controlled. Excess sialylation of a cell may be bad, but specific over-sialylation of EGFR could turn out to be very useful, if we can learn how to modulate and target it. Another common example of this phenomenon is radiation therapy. Normally, high levels of radiation are bad for your body, and can actually cause cancer, among a range of other illnesses. But, there are many different ways of specifically targeting radiation to a tumor to kill it while sparing (to varying degrees) the surrounding tissue and the rest of the body.