Look at how the letters are signed!
Here's the explanation as printed in a Haggadah with the commentary of the Vilna Gaon:
However, the February 21st issue of that same year printed a response, which defends the Vilna Gaon:
I'm not precisely sure who "N.L.D. Zimmer" was, however he seems to have been one of the founders of the Federation of Synagogues, as well as an inveterate letter-writer of the period. It is supposed that the character Karlkammer in Israel Zangwill's Children of the Ghetto was based on him. Here is a passage from that book which would give a flavor of him:
Near the central raised platform was a group of enthusiasts, among whom Froom Karlkammer, with his thin ascetic body and the mass of red hair that crowned his head like the light of a pharos, was a conspicuous figure.I saw elsewhere that on the occasion of Moses Montefiore's 100th birthday, "Mr. N. L. D. Zimmer sent a long letter in Hebrew, which contained innumerable quotations from the Bible, all commencing with the letter ק."
"Peace be to you, Karlkammer!" said Pinchas to him in Hebrew.
" To you be peace, Pinchas!" replied Karlkammer.
"Ah!" went on Pinchas. "Sweeter than honey it is to me, yea than fine honey, to talk to a man in the Holy Tongue. Woe, the speakers are few in these latter days. I and thou, Karlkammer, are the only two people who can speak the Holy Tongue grammatically on this isle of the sea. Lo, it is a great thing we are met to do this night — I see Zion laughing on her mountains and her fig-trees skipping for joy. I will be the treasurer of the fund, Karlkammer — do thou vote for me, for so our society shall flourish as the green bay tree."
Karlkammer grunted vaguely, not having humor enough to recall the usual associations of the simile, and Pinchas passed on to salute Hamburg.
Below is a letter of his printed in Notes & Queries in 1881, regarding the origins of the cry "Hep!" or "Hip, hip hurrah!":
He seems to have had quite a גמרא-קעף:
Below is an amusing letter from someone who didn't like him very much, where he is accused of having pretensions of being Rabbenu Zimmer, trying to keep everyone married to two יקום פרקןs:
However, does anyone have the complete explanation as to why the Gaon would seem to state that as a rule לילה is לשון נקבה?