I would imagine that Pastor Russell would say what all Jehovah's Witnesses would say today, that the Bible is the truth, that their religion is in fact Bible based and that the Bible is the authority for their doctrine.
However Bible-based actually means nothing significant and the claim that it is the authority for their doctrine is demonstrably false. In reality very doctrine which is unique to Jehovah's Witnesses is extra-biblical; established solely by eisegesis sprinkled with theories and speculations that are beyond what is written.