While I think that's probably an over estimate of their sales, they were like this crazy shooting star.
They were only really big for maybe 6 or 7 years or so, but in that time it's hard to overstate their popularity. Truly a global phenomenon, just in the US I think they had 20 charted songs in those few years.
And in the subsequent years you had the Abba Gold compilation that kept their hits around, and then the Mamma Mia movies and stage show that not only kept them alive but introduced them to a new audience.
The fact as mentioned above that they were offered a billion to reunite tells you all you need to know about how big they were. Heck, even only playing for those relatively few years, each of their net worth Reno's estimated north of 200 million.
Just crazy.