Is $\{a^nb^n\}\cup\{a^nb^{2n}\}$ LR(k)?
Propbably you should read the citation as: "The problem is that the c (which separates the two parts of the language) is at the extreme right and not at the extreme left, where we start parsing."

Therefore it is of no help (unlike the case where you start reading at the right), and therefore there is no LR-grammar for this language. Neither with the c nor without.