The read I'm getting is that the way that Sony is handling BC is fundamentally different from MS. MS is doing it in software, whereas Sony is trying to do it in hardware by downclocking. The slide where they showed PS5 Native, PS4 Pro Legacy, and PS4 Legacy are probably performance profiles for the system - most games work with those profiles, but because RDNA2 and Zen 2 *aren't* 1:1 with GCN and Jaguar, there will be some issues with compatibility on some titles.
Nowhere did I get the read that Sony or Cerny were talking about Boost Mode. They never mentioned it as a possibility and they didn't imply it - it sounded like, to me, what I just said - because the hardware isn't quite the same, and will inevitably be faster in some instances even in those compatibility modes, there will be issues in some games that they will have to test for. They've currently tested 100~ and they mostly work, but they can't commit to making sure all of them work at launch, and they'll be testing and adding support for more games over time (read: post-launch).
I'd love to be wrong, but it's a bad look when we can't even be sure we'll be able to play Bloodborne @ 1080p on PS5 when MS is showing HDR/4K OG Xbox games day 1 on Xbox X, along with 100% XB1 compatibility + enhancements.