The __backup_config function sets a log_only return variable based on values in a target list (specified by in the config of the backup cursor). The relevant code is basically:

   log_only = false;
   target_list = false;
   for (target_list = false; __wt_config_next(...) == 0; target_list = true) {
       if (WT_PREFIX_MATCH(key, "log:")
           log_only = !target_list;
    }

We walk through values specified for "target" in the config, i.e. "target=(file:foo.wt,file:bar.wt,log:xxx.log)"

The way I read this, if the first item in the list starts with "log:", and there are no other items that are "log:", then log_only will be true. This log_only value influences whether we have a "WiredTiger.isrc" file vs. some other files in the backup. I don't know how "target" is expected to be used, but this seems strange, and the comments do not enlighten.