The WT from the time of the great plagiariser Charles Russell has always read and plagiarised the writings of others, I would suggest there is more than a distinct possibility that Herb Armstrong was similar, taking on board for his own use whatever he felt would work, a two way cross fertilisation of ideas.
They are both offshoots of Adventists. The Church of God that Armstrong joined was formed before Seventh Day Adventists, and they have in common the Adventist teachings, but they rejected the teachings of Ellen G White.
Russell was influenced by Adventist teachings, and borrowed heavily from them. It is likely that he was drawn to their rejection of hell fire doctrine, which was almost an obsession with him.
When you add in some policies from the High Control Group Handbook, it can make both groups appear a lot alike.