As is often the case, the title is hyperbolic. The discovery applies to 20% of tumors, and "one of cancer's significant defenses" or "a key weakness of cancer" would be more accurate.
That said, I'll happily take "we discovered a key weakness in 20% of cancers," please and thank you.
That said, I'll happily take "we discovered a key weakness in 20% of cancers," please and thank you.