I don't think the author's math is wrong here. Looking at this sentence, he/she is not taking the average for both like you described, but applying growth rate

I don't think the author's math is wrong here. Looking at this sentence, he/she is not taking the average for both like you described, but applying growth rate to revenue for each individual company: If we applied the companies’ monthly growth rates to their reported revenue, then after just one year the 22 companies would be generating about $21 billion in combined monthly revenue, or $963 million monthly revenue per company.