It just mean they need to test the game on PS5 before ship.
In simple terms:
+ Old PS4 titles: Sony will test themselves.
+ New PS4 titles: Devs needs to tests before ship the game.
As Cerny said and that article says, it's:
-Old PS4 titles: Sony tests to certificate it works in PS5 BC mode, if there are issues contact the devs of this game to suggest them to fix it. If needs fixes and the dev doesn't fix them, the game is not supported by PS5 BC.
-New PS4 titles, Dev tests and after thats Sony tests to certificate. Dev must fix the issues to publish the game
Sony expects to have over 4000 PS4 games being supported by PS5 BC, which means almost all the PS4 catalog, which has over 4000 games. Maybe some dev who released a game while ago may not be able to fix it because the company shut down, or don't work with that publisher anymore, or they don't have the code anymore, or are too busy with other projects, or they think it isn't profitable to spend a budget to fix this.
Damn, the XSX is potentially launching with a much larger library then.
Looking at how BC did work at 360, or how it did work in One, we'll have to see how it did works on Series X when released and what compatibility list has. Because remember that for the streaming of the other day they were talking about gameplay again and again, and saying stuff like there was going to be a step like from 2D to 3D and things like that.
As of now listening the Series X BC PR talk sounds great, but let's see what they actually deliver in the final product.
so does this mean the rumours of 100 back combat games at launch could be true? I mean why if a game works on PS4 should it not work on PS5 ?
They are not a rumor. Sony said that they are testing every single PS4 game to see if it works on PS5, because like in all BC there are always some games that cause issues if not tweaked. Sony aims to have over 4000 games, which means basically almost every single PS4 game in the market to run on PS5. For different reasons, some devs may need time to tweak/fix their games. so some of them may be end in BC but not at launch.
Cerny said that out of these thousands of games that are being tested to run well on PS5, just before his conference they already tested the 100 most played PS4 games and most of them were already ready to run on PS5 at launch. Then Sony clarified that these 100 games weren't only the PS4 ones ready to run on PS5 at launch, that they were going to have a ton more with the idea of having mostly all the PS4 catalog runngin on PS5.