A few things
The Unreal Demo wasn't for marketing. It was part of a developer communication to invited parties that very much was not supposed to become public. Nintendo is likely trying to genuinely show what the hardware can do.
Yes, it seems like it was running on non-NIntendo hardware "specced" to the performance target. But you can go buy a 3050 right now and get an Ampere GPU very close to Drake's design. You can get Orin with an Ampere GPU and and ARM CPU. It seems likely that you could get such a machine really close to final performance.
As Thraktor and others point out, yes, there will be tuning up till late in the production process as battery life and thermal properties get dialed in. But unless there is a serious fuck up, that's unlikely to be a gigantic change from the target.
This is almost definitely not stock Unreal on an underclocked PC, with someone tweaking settings. It's a beta of Unreal running on Nintendo's software stack, NVN2. This isn't just Nintendo demoing hardware, it's them demoing the most popular gaming engine in the world, ported to their stack.
"We have full Unreal 5 running. Ray tracing is running using our RT pipeline. Nanite is here, using mesh shaders and asset streaming from disk. Mass AI is running for the NPCs, so no worries about these mobile CPUs. Full fat DLSS is integrated for resolution scaling. And rather than just tell you we have these things, let me show you, so you can see that when I say 'acceptable framerate' you can see I really mean it."
Such a demo is designed to answer all the detailed questions faster than a specc list. If you've been in this thread for a while, you know "but can DLSS 2 really do that" or "you think they're going to add real RT in 10W of power?" come up a lot. It's not arguing about the spec, it's asking how far can the spec deliver, and what is it going to look like when dialed down that far.