Not sure if I understand correctly, but wouldn't one of the two "operands" need to be a file in any case (the linked tools would require you to paste two times

Not sure if I understand correctly, but wouldn't one of the two "operands" need to be a file in any case (the linked tools would require you to paste two times as well)? In that case, assuming that one file is OK and only the other text needs to be pasted, you can diff against stdin. On macOS there is `pbpaste`, which prints the contents of the pasteboard to stdout. This allows you to do `pbpaste | diff the-file -`, with `-` being the commonly used "filename" for stdin.