You spelled "misspelled" correctly, which earns a point in my book because (in a profound display of irony) a lot of native speakers misspell that one!
“Misspelt” is the usual UK/Commonwealth spelling, if that’s the spelling you’re referring to. “Misspelled” is the usual North American variant. Both are correct!
Not the person to whom you’re replying, but I think they mean many English speakers misspell the word with only one “s”: “mispell” (sic), “mispelled” (sic), etc.