Copy pasting into the browser to view the link triggers this, just copying doesn't. If I understood the previous comment correctly, the author copies links into the browser instead of clicking them (since they aren't clickable links).

"Copy/paste" here means "Copy the URL, paste it into the address field of a Web user-agent program, and initiate navigation to that URL in that Web user-agent program".

I suspect the phrasing was used since the discussion is about text-only email and, this being Hacker News, someone might have nit-picked that they view plaintext emails in a mail user agent program which does not scan for strings matching the format of a URL and offer a direct way to click on them or otherwise trigger automated opening of them in a Web user-agent program, and so the person who wrote "copy/paste" had to avoid saying "click on the link".