I think the original poster corrected the years in that the study was predicting 28 million witnesses by 2090, and that the study is ten years old or more. 2090 is a long way away (at least a couple of those overlapping generations they keep talking about) and a lot can happen in the world in 77 years. If there are 7 million now, with a prediction of 28 million in 77 years, that implies a 1.8% growth rate. At the same time, the world population which is about 7 billion now, and lets assume a 1.1% world population growth rate (the figure given for 2012 in Wikipedia), in 77 years the world population would be about 16 billion.
Compounding makes the figures look scary, and the assumption of growth rate really makes a big difference when compounding over such a long period. If either the government opens up in China, or if cultural and religious attitutes change in India, Pakistan, Bangladesh, etc, the 28 million could possibly happen. I find it doubtful though.