An important thing to keep in mind is that both the Old and New Testaments of the Bible encourage people to pray to Yahweh/God and to ask for his guidance and insight. In addition, the New Testament says that the Holy Spirit is available as a helper and that followers of Christ should pray for Holy Spirit. As a result many devout Christians (probably many tens of millions), including people of numerous denominations and sects and those who are nondenominational (or even independent Christians), pray to God, Christ, and/or the Holy Spirit, and believe God, Christ, and/or the Holy Spiri in some way gave them ideas. It is thus natural that some of the writers at the WT would also believe and/or say that Jehovah God is leading them and in some way giving them interpretations of the Bible. Likewise numerous people of so-called pagan religions believe that some God, god, gods, spirit, spirits, and/or force is(are) giving them ideas.
That probably explains why there were so many people in Bible times (according to the Bible) who were false prophets. Namely (at least from an atheistic philosophical naturalist point of view), so many people sincerely (but incorrectly) thought they were getting ideas from the God, a god, a spirit, or spirits when instead it was their own human mind that gave them the ideas. [In other cases people were outright lying, not believing themselves were prophets.] I've talked to a number of individual Christians (of various churches/sects and those of no church/sect affiliation) who tell me that God has told (or in some other way conveyed) ideas to them. A great many people are superstitious (an atheistic point of view considers it to be superstitious) in this regard, not accepting that their own minds could get such ideas independent of some spirit being.
A lot of religious literature (and verbal instruction) encourages such superstitious thinking, by telling people that supernatural beings (sometimes a being deemed honest and benevolent and sometimes one deemed the opposite) insert thoughts into their minds. People are encouraged to engage in thinking patterns and practices which are claimed to invite a good God/god/spirit to communicate with them. Likewise people are encouraged to avoid thinking patterns and practices which are claimed to invite a bad God/god/spirit to communicate with them. Even the fictional Star Wars movies have a theme of encouraging the influence of the good side of the force and discouraging the influence of the bad side of the force.
Our brains naturally cause ideas to jump into our conscious minds randomly (or at least seemingly randomly). Sometimes I get ideas that seem to come to out of nowhere (and having no relation to what I was consciously thinking about at the time) and which also seem so brilliant to me that is hard for me, even despite being an atheist, to believe they are my own ideas. I convince myself that some part of my subconscious mind thought up the ideas (perhaps thinking about them for time) and presented the ideas to my conscious mind. I can thus see how devoutly religious people, especially ones imploring God for guidance, could be convinced they were getting ideas directly from a supernatural being (whether a good one or a bad one).