- EUV is not unknown to the Chinese technicians working on it.
- DUV already exists in China.
- The major differences between EUV and DUV is still the light source.
The theory is public, but the devil is in the details. Knowing the public details would be enough to speed up research somewhat but EUV is way, way, more complicated than DUV. There are no EUV machines in operation in China they can examine either.
DUV already is available for inspection in China but EUV is totally different. And you can't say it is just a matter of the light source. The way the light source itself operates is totally different from DUV. You need to hit tin droplets in flight with laser pulses. The way the light is focused is totally different with mirrors instead of lenses, once again a different skillset.
The mask materials and the resist aren't the same either.