They were careful with how they phrased it, leaving the possibility of a refresh without a performance uplift still on the table (as speculated by media). It looks like the OLED model's core performance will be only marginally better due to faster RAM, but that the APU itself is the same thing with a process node shrink (which improves efficiency a little).
See also: PCGamer article about an OLED version. They didn't say "no", and (just like with the previously linked article), media again speculated about a refresh happening.
It looks like they were consistent with what they were talking about with how it wasn't simple to just drop in a new screen and leave everything else as-is, and used that opportunity to upgrade basically everything a little bit while they were tinkering with the screen upgrade.
Sure, but not much of that battery improvement is coming from migrating the APU's process node. Moving from TSMC's 7nm process to their 6nm process is only an incremental improvement; a "half-node" shrink rather than a full-node shrink like going from their 7nm to their 5nm.
The biggest battery improvement is (almost definitely) from having a 25% larger battery (40Whr -> 50Whr), with the APU and screen changes providing individually-smaller battery life improvements than that. Hence the APU change improving efficiency "a little".
It's smart. Also, developers have a solid benchmark to set their games to. Console has long had the benefit of a stable hardware set over the course of many years, which makes it easier to develop to the broadest possible market. Skipping incremental APU updates has a benefit of keeping a longer benchmark for game developers hoping to boost sales by targeting the market with handhelds. Valve was pretty clear in their communication in this regard, which is great.