Which particular claims? The entire last third of the article is about the decrease in dimorphism. The explanations begin on page 444, near the end of the page.
Also see:
http://kuscholarworks.ku.edu/dspace/bitstream/1808/124/3/annurev.an.14.100185.pdf
Notice how things like better hunting technology were accompanied by a decrease in male musculature and physical robustness. Sexual dimorphism exists because of evolution in response to particular selection pressures, but when those pressures disappear, nobody's being selected for based on those pressures, and they start to homogenize through the pull of regression to the mean.
Also notice that the generation decline in male testosterone is not regarded as being explained by simple lifestyle changes like smoking or eating differently, which suggests a longer-term, more subtle explanation:
http://www.healio.com/endocrinology/hormone-therapy/news/print/endocrine-today/{ac23497d-f1ed-4278-bbd2-92bb1e552e3a}/generational-decline-in-testosterone-levels-observed
Anyway, regardless of the explanations, dimorphism has been decreasing, and women and men have been becoming more similar in those ways for thousands of years, and there's no reason to assume this process won't continue in the future.