Rev 20:5a is a phrase that has invited debate for centuries. Manuscript evidence is pretty much a wash as the issue was already in debate before the earliest extant manuscripts. The best suggestion recommending it's inclusion is a simple scribal error. His eyes dropped from 1000 years in one sentence to the 1000 years in the next verse. It's logical and typical of such errors. Since manuscripts are a wash, as I said, the motivation to include or exclude largely stems from the theologian's take of the book's eschatology. Almost no one comes away from the book with the same understanding. If the work is to read on it's own terms it doesn't agree with the eschatology of the WT nor mainstream Christianity. It is pretty much exactly the late 2nd temple Jewish eschatology found in sister works of the period.
The wording is a bit clumsy but it may have reflected the author's own explanatory note. Who knows?