The WT's explanation is: "The Christian congregation had not yet been set up, so his not being part of their group did not mean that he was of a separate congregation." February 15, 1988 WT, p. 8. -- Ding
In line with the above, in 1967, an anointed brother explained that, initially, the arrangement was in flux, but as time progressed the structure of the congregation became tighter and, as a consequence, more restrictive. He quoted Matthew 12:30:
"Whoever is not on my side is against me, and whoever does not gather with me scatters."
Of course, what he told me doesn't address the variables inherent in such a simplistic explanation. I was yet an unbaptized student and that answer sufficed.