A WT answer would be because they had honest hearts and were seeking the correct doctrines although they had not yet worked it all out.
It would appear that the WTS heads weren't seeking out bible truths in reflection of the organization persistent Jesus has returned (1914) Armageddon soon doctrines which were proliferated all throughout the 20th century.
They had the true heart of commercialization comprised of building up their own power and money as an established religious publishing house, the proclamation of them being god's selectively chosen ones was just a self supporting marketing gimmick or strategy to lure people to their literature and the organization.
Want to understand and truthfully interpret the bible the word of god , come to us and will show and tell you, its all in are literature.