That's not always true. In some cases adoptions can be vacated. They are not always permanent.
If you are referring to some provisions in the CA Family Codes that pertain to Indian Tribes, and the adoption process, it seems you might be correct. Otherwise; the CA Codes [from my interpretation] seem to support the information offerered by Gemini47.
Excerpt from: California Family Code 9102. (Bold lettering added to highlight key points)
9102. (a) An action or proceeding of any kind to vacate, set aside,
or otherwise nullify an order of adoption on any ground, except
fraud, shall be commenced within one year after entry of the order.
(b) An action or proceeding of any kind to vacate, set aside, or
nullify an order of adoption, based on fraud, shall be commenced
within three years after entry of the order.
Source: CA Codes (fam: 9100 -9102)
I'm not a lawyer. This is only my opinion /suggestion. Most Replys' are based on information provided by the "original post" (OP).