I don't know what a "CST" is, but being in class is not the same as providing socialization skills. It is providing socialization opportunities, but for a child who doesn't already know how to use those opportunities successfully, that is not adequate. Most children on the spectrum require direct instruction in socialization skills. The goals vary, depending on where a child is, socially. For some children, the goals could be using a conversational partners name prior to speaking; turn taking in a "conversation" so that there are two or three "turns" by each partner in the conversation - but then you need to have something like whether there are prompts, whether the conversation takes place in a structured or unstructured environment, how many times such a conversation occurs ...; responding to greetings (verbally or otherwise); maintaining eye contact. Sometimes a curriculum to teach idioms and flexibility is necessary.
The social skills and autistic supports provided by an OT or SLP are NOT speech or OT support in the traditional sense of those disciplines - it is just that those professionals tend to be capable of providing the service competently.