They are not correct.
They are wrong ? No, both are right!
Basically, undecimate is a pun, a clever pun at that, imho!
Language relies on context, is rich and ever evolving. The word "decimate" can mean any of Websters definitions. The word decimate comes from the latin decimus, a tenth, and undecimus means, you guessed it, an eleventh.
El'Reg cleverly took one meaning of decimate, noticed an off by one error and hence came up with undecimate, which, ironically, also means the contrary -> El'Reg loves these pun's and so do I, one of the reasons I read their articles ... the other being above average technical boffinry in the tech journalism world and their cos^Hmic units of measure!