He know it was a 747, he clearly states he saw two rows of windows. Only the 747 had that feature at the time. RC-135s only have a couple of windows total. Any interceptor pilot would have gone through aircraft recognition. Military aircraft that are intentionally violating enemy airspace generally try to not make it easy to locate them.
I would not say the USSR acted any better than the US did, they were intent on shooting KAL007 down even if it reached international water.