I don't think they knew how good the BC would be in March, when Cerny mentioned that it worked well for the top 100 games. The 99% claim came much closer to release, when they'd done more extensive testing and given developers a chance to patch problematic games. Perhaps they should have been more clear from the outset that they'd expected a high degree of compatibility across the entire library, but I think they clarified that rather quickly (without putting a number on it, since they didn't have one yet.)
Not true, they updated the article covering Cerny's talk in March iirc.
Unveiling New Details of PlayStation 5: Hardware Technical Specs [UPDATED]
Watch live for a deep dive into PS5's system architecture and how it will shape the future of games.blog.playstation.com
What's not true? I mentioned the clarification you linked to, and wrote that it came quickly. But it didn't contain the 99% claim, which came much closer to launch, in September:
PS5 will be ‘99 percent’ backward compatible with PS4 games
PS4 games could give you something to play as the PS5 lineup fills out.
www.theverge.com