But given that our term 'the evening star' actually denotes a planet, perhaps we misdescribe Mary's belief by employing this term.

I believe that is exactly what Eliezer is saying.

"The evening star" refers to two entirely different things to Marry and John. John believes 2+2 = 4, while Marry believes 2+2 = red and 4 = blue. Attempting to substitute 4 for red does not work, because John's 4 is not even the same type of thing as Mary's.

John learns that Mary believes 2+2 = red. When John sees Marry write ((2+2) + 4) = purple, John incorrectly thinks Mary believes (red + red) = purple.

The problem, of course, is that Mary does not believe 2+2=4, to her that would be ridiculous, so John makes an incorrect inference about what Mary does believe because his beliefs are entirely different.