It's possible the author was well aware that "Zulus" would have been correct for the Army phonetic alphabet, but also realized that could have been perceived as a rather problematic name to use...In the book/movie World War Z, the soldiers call the zombies "Zack." I guess it was supposed to be like Vietnam era soldiers calling the enemy "Charlie." Sure, soldiers in a zombie apocalypse would come up with a nickname for zombies - but that is objectively the wrong one.
The nickname "Charlie" for the Vietcong comes from the Army phonetic alphabet. In the phonetic alphabet the letter V is read "victor" and C as "charlie" (to avoid miscommunications over poor quality radio transmissions). Vietcong got abbreviated as "V.C" which then expanded to "Victor Charlie" or just "Charlie." In the Army phonetic alphabet Z is not "Zack" it is "Zulu." So in an actual zombie apocalypse the army nickname for zombies would obviously be "Zulus." Zack is just stupid and not something any soldier would have come up with.
Dustin's collection of Transformers in one episode also includes toys a year earlier than they actually appeared. Suspect there's lots of little issues like that. There's also the whole implication that the "Upside-Down" was official D&D lore...Oh, and Stranger Things:
1) That’s not a 1970’s-80’s Millennium Falcon in Stranger Things. The engine stickers are wrong.
And,
2) The Bangles’ Hazy Shade of Winter was 1987, and also had no business closing an episode set in 1983.
Great song, and great show though.