Can I get a China visa without an Alien Registration card from Korea?

You can try to apply for a China tourist visa through Chinese consulate in Pusan, by any a travel agency appointed by it. Or, you can send back your China visa application to Netherlands if time permits. Another option for you is to apply in Hong Kong if your travel plan allows a change to visit Hong Kong first before visiting mainland China.

Thank you for your answer! Is it guaranteed that I can get my visa in Pusan? Or is there a possibility I cannot get it in Pusan either?

If you don't have an Alien Registration card from south Korea and this is your first time to China, you won't get a China visa in Korea. That is for sure.

Today a travel agency in Pusan told me they can make it, so I traveled to Pusan today. Is it not true?

You can go to Busan to have a try, the visa policy is not stable, when the new visa rule "non-Korean applicants applying for China visa in Korea have to provide Alien Registration with at least 6 months validity began to apply, China consulate in Busan can still process the application from non-Korean applicants without ARC. A few months later, the consulate stopped to accept such applications. Now, maybe it starts to accept again.