Really confused here - batch jobs are allowed 1 callout execute(), and I made 1, and then got this error.

I set the batch size to 1 as well.

In fact, I basically copy & pasted a different batch job where I'm doing something very similar, with the only change here being I'm using the standard  "implements Database.Batchable<sObjects> " instead of a custom iterable.  Any thoughts?

global with sharing class g2wGetRegistrantDetailsBatch implements Database.Batchable<sObject> {

   public String sObjectQuery ='Select Id, CampaignId FROM CampaignMember' ;
   
   global Database.QueryLocator start(Database.BatchableContext BC){
        return Database.getQueryLocator(sObjectQuery);
   }

    global void execute(Database.BatchableContext BC, List<sObject> scope){
        //Process this 1 campaignMember.  The batch size is always 1 due to 1 callout / batch job limit
        CampaignMember cm=(CampaignMember)scope[0];
        Campaign campaign=[SELECT Id FROM Campaign WHERE id=:cm.campaignId LIMIT 1];
        String requestURL='<my API request URL>';
        
        try{
            //Get registrant details
            system.debug('The request URL is'+requestURL);
            HttpResponse response=g2wUtils.getResponse(requestURL);//send out the request to the other server
        }catch(DMLException e){
           
        }//try
        
    }//execute

    global void finish(Database.BatchableContext BC){
   
    }//finish

    
}//g2wGetRegistrantDetailsBatch

 Called by:

    public static void scheduleQueuedBatchJobs(){
       g2wGetRegistrantDetailsBatch job=new g2wGetRegistrantDetailsBatch();
       String query='Select Id, CampaignId FROM CampaignMember';
       job.sObjectQuery=query;
       Id getRegistrantDetailsBatchJobId = Database.executeBatch(job, 1);//need the batch size at 1 due to callout limit of 1 per batch
    }//scheduleQueuedBatchJobs