Although Armstrong denied ever being a Witness or being influenced by them (he claimed his only influencer was God, through the Bible), the similarities between the two are just too great to ignore. Witness folklore places Armstrong as a former Witness or at least a Witness copier, but this piece of evidence shows that at least in this case, the copying was going in the other direction.
There is no possible way that two organizations could simultaneously predict the end of the world for 1972 and 1975 - both of which dates are based on the same chronological errors - without some cross polinization. I suspect that both Franz and Armstrong regularly followed each others rantings, and copied the juiciest bits.
I would call it mutual plagarization.