Waaaaaaay back I worked for a magazine/publishing entity and was told by the publisher that a national “best seller” sold 500K books. That was in the 70s, pre-internet, pre-Kindle, etc. So maybe S&S was just hoping for an inside track, a post-WH Hillary book? A Bill book? Or a discreet political contribution? Who knows?
You were lied to. Only books like the Bible sell in the millions on a regualr basis. The previous comments noting 15 to 20K per week are correct. It does matter what time of year book is released. Christmas has higher sales. The only part moving of this dynamic is that there are about 100 million more people in the US since the 1970s. Thus more potetial readers.