Re: I'm not surprised...
《Unsurprisingly, searching directly for that quotation to confirm it didn't find a hit; mostly I got online grammar checkers, or (with quote marks) nothing at all. I had to search for a libretto for Iolanthe and then search that...》
DuckDuck quacked on
Gilbert and Sullivan "I can spell all the words that I use, and my grammar's as good as my neighbours'."
on the third entry after two adverts (grammar training, then the usual ebay bait [& switch] :)
https://gsarchive.net/iolanthe/web_op/iol08.html
I am surprised that Bing wasn't too shabby at this, particularly given it's a Microsoft creation.
Looking at the libretto, some of the versification sails pretty close to doggerel. Tabors/Neighbours, Hearty/Party - end rhymes to die for, or do I mean lethal? If I were Gilbert I mightn't set my hopes on Poet Laureate (actually he wouldn't have been the worst.)