It means that if you trace Y-chromosomes back 5000 years, you'll find that grand-grand-grand-...-fathers of 40% of people are concentrated in the area of the Yamnaya culture. Grand-grand-grand-...-mothers would be from multiple groups, yes.
r1b's population is only 190 million [1]
[1]https://www.razibkhan.com/p/the-haplogroup-is-dead-long-live...
> r1b's population is only 190 million [1]
Their population likely had multiple haplogroups. R1b was the most common (5000 years ago). But, yeah, the 50% number looks too high.