The footnote refers to Schröder's work. I'd appreciate if someone can explain Schroder's insights and spare me some hard reading.
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Schröder says in his book that this formula looks paradox on the first glance since there is a conclusion $r$ on the right side while $r$ does not occur on the left side of the formula. The oddity is that the premise on the left side knows nothing about the statement $r$.
More literally: Very paradoxical at first glance the universal pair of formulas of propositional calculus:
Paradox because the statement $c$, over which the right conclusion reported in the left premise does not occur, it can thus contain no information on that statement, while is nonetheless entitled to conclude from the statement left-hand to refer to the right hand "deductive". (http://www.deutschestextarchiv.de/book/view/schroeder_logik0201_1891?p=294)