It has already sold insane numbers of copies on pre-order.

https://ps-store-db.live/games/10000730

It’s been consistently in the top 5 selling games on PSN, across every major territory except Japan, for the entire duration of its pre-order(2 months). That’s longer than big AAA games will spend in the top 5 during actual launch, which is maybe a couple of weeks. A second tier AAA game like Onimusha never made the 5 in any major market except Japan(Peaking at 8 in the USA and UK)

Compare that to the pre-order numbers for a normal game like Phantom Blade Zero

https://ps-store-db.live/games/10000730

It is yet to make the top 10 of any of the major territories for even a single day,

Can't believe it's three weeks till release.

https://youtu.be/loN2NIjl_3s?is=qVQKW_UHdlhXtInp

lol

Older games also get released/remastered on digital storefronts and they would benefit with BC too. 

Although I don't see how this would fudge sales numbers to a significant degree. Just because older games re - release for multiple generations doesn't mean they will sell anywhere close to the original. GTA V is the obvious outlier. Remakes like TLOU Part I and Ocarina of Time are other obvious outliers. But sales charts wouldn't count them as the same SKU anyways. People counting all the versions of say FF7 (PS1, PSN rerelease, PS4 remaster, etc) as one version is an entirely different story, and usually only done for enthusiast forums like this. 

I'll give you an example.

According to the internet, SMB3 sold around 17 million copies on NES, and Minecraft has sold around 425 million copies. Now, looking at those numbers, if you weren't around when SMB3 came out, which is plausible, given its age, you might laugh if someone said that SMB3 was just as big, if not bigger than Minecraft was back in the day. But it was. It totally was. And so was GTA on PS2 (any one of them), MGS 2, Halo 3, Wii Sports... so many titles. But if you look at their numbers, it's like... yeah... right. Because the record books have all been muddied up due to changes in the industry. 

I don't know who is saying a billion copies but it will certainly make a billion in 24 hours.