The chip might be set, but that doesn't mean that Nintendo has set every aspect of the console overall. Nintendo expanded memory on the Switch, famously,
but also upped handheld clock speeds very late in the pre-launch development period. The NX was a famously fast turnaround, so it seems likely that Nintendo would have nailed those sorts of details down before sending "final" devkits
The finalized devkit for the Switch was a pretty polished unit.
You can see some examples here. You'll note that the body looks very much like the Switch, but it's clearly also a custom design to support the extra ports in the back - it was clearly designed in tandem or even after the final body for the Switch itself was designed.
You can also see that it comes with a "cradle" to dock it to the TV, because the unit is too large to fit in standard dock. Early descriptions of NX devkits didn't have a screen/dock - developers were pretty sure, in fact, that the "handheld profile" was a separate piece of hardware entirely, more like a Wii U.
Since I'm pretty set on believe holiday 2024 is the launch window, final devkits seem unlikely, but putting that aside, it's entirely possible for the SOC to be done, but the devkits to not be quite final.