Welcome solafide,
As far as I can remember even the WT explanation is not quite that simplistic: they acknowledge that the absence of definite article is first of all a matter of syntaxical function: theos as predicate does not require the article, whether it refers to "God" or "a god" (they consider the latter option grammatically allowed, not required; whether it makes sense in the context of Johannine theology is another matter). An anarthrous predicate noun is qualitative, expressing what rather than who the subject (here the logos)is (a subtle but important nuance).
Other examples of similar constructions in the Johannine context (you'll note that the English translation may require the article although there is none in Greek) may help you grasp what an anarthrous qualitative predicate is:
3:6: What is born of the flesh is flesh(sarx estin), and what is born of the Spirit is spirit (pneuma estin).
3:29: He who has the bride is the bridegroom (numphios estin).
4:24 (with estin implicit): God is spirit (pneuma ho theos).
5:27 because he is the Son of Man (huios anthrôpou estin).
6:63 The words that I have spoken to you are spirit and life (pneuma estin kai zôè estin).6:70 Yet one of you is a (the?) devil (diabolos estin).
17:17 your word is truth (alètheia estin).
1 John 1:5 God is light (phôs estin).
4:8,16, God is love (agapè estin).