A couple of things here...... All of them good.
#1 Being an SCWI is not a requirement of AWS D1.2 for writing welding procedure specifications.
What B5.1 is trying to communicate is this: (paraphrased) In order to be an SCWI, one is expected to have the ability to generate a PQR. Elements of this requirement are part of the exams. A CWI or a non-certified person can also make a PQR. It's just not an expectation.
Having said that: Writing a PQR for AWS D1.2 can be difficult, so having a consultant or an experienced SCWI to help you get through the first one might be a very wise investment for you and your company. Really learning how to do it right. On the other hand, you may have all the experience required, having a full understanding of aluminum welding and the code you wish to implement. But if you just bought the code book, having a mentor for the first shot sounds like a good investment.
So unless you have some specific contract/customer requirements that state specifically that an SCWI is to be used to make the Procedure Qualification, than you or anybody else (other than a CAWI) can do it.