You know that the terms Win32 and x86 are orthogonal? One is a processor architecture the other one is an API. Both processor (ARM/x86) architectures are supported by all APIs (Win32, UWP, .Net/CLR). The only question which needs to be answered when it comes to emulation is not if the program is Win32 or UWP, but if it is an ARM or x86 assembly/executable.
In summary, claiming that Win32 programs need to be emulated is wrong, claiming that x86 programs need to be emulated is correct. The API has not the slightest influence on the question of emulation.
Regarding the press conference, i cannot rember Microsoft saying something about ARM optimized with respect to Windows 10x as the only device they were showing with Windows 10X running was an x86 device. Besides what else could they possibly optimize with respect to ARM? Their emulation technology in Windows 10 is already world class.