So, there are two things here:
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highlighting text in a closed topic and choosing Reply as linked Topic will quote the whole post instead of just the highlighted part. This is a bug if it should be as @codinghorror suggested. On the other hand, a link to the right of a post indicates it’s about the whole post. While a button next to highlighted text is clearly only meant for that text.
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everywhere on Discourse you can select text and a button to quote will appear. Not on closed topics. This is counter intuitive