For your convenience here is the interview he’s going over in the video: https://www.nintendo.com/us/whatsnew/ask-the-developer-vol-16-nintendo-switch-2-part-4/

An interesting part is this:

Dohta: If we tried to use technology like software emulators, we’d have to run Switch 2 at full capacity, but that would mean the battery wouldn’t last so long, so we did something that’s somewhere in between a software emulator and hardware compatibility.

Sasaki: This is getting a bit technical, but the process of converting game data for Switch to run on Switch 2 is performed on a real-time basis as the data is read in.

Is it like having Switch games “simultaneously translated” for Switch 2?

Sasaki: That’s right. […]

So it sounds like they are doing some recompilation. /u/jonathansmith14921 had an interesting comment over on reddit, his suggestion is that they have to recompile the shader bytecode from Maxwell to Ampere to fit the new GPU. Makes sense to me.

Another interesting titbit from that thread, there are official (in)-compatibility lists: Launch-able but has issues and has issues even launching