Creating several Google Calendars and avoiding 403 : Rate Limit Exceeded

Question

I need to create up to 130 calendars on a Google account using Java and Google Calendar API but keep getting

"403 : Rate Limit Exceeded".

What I've tried :

-looping with service.insert(calendar).execute(); -> result : I receive error 403 after 25 inserts ( which weirdly enough seems to be the old limit, should be 60 according to : https://support.google.com/a/answer/2905486?hl=en )

-looping with delay between each request (up to 60 seconds) -> result : Didn't change the outcome. still 403 after 25 inserts ( In its documentation about Exponential Backoff, Google talks about mere seconds so I should think 1 whole minute is enough even if I don't increase exponentially that delay ).

-Request Batching ( following THIS Google example code ) -> result : After about 10 callbacks the response falls into the onFailure method with.. You guessed it 403 status code.

I think I am well within my ( maxed-out ) API quotas most of the time. I've seen "quotaExceeded" only once after several tests.

Batch Request sample :

batch = this.service.batch();
JsonBatchCallback<Calendar> callback = new JsonBatchCallback<Calendar>(){
    @Override
    public void onFailure(GoogleJsonError e, HttpHeaders responseHeaders) throws HttpResponseException{
    log.debug(e);
    throw new HttpResponseException(Integer.valueOf(e.get("code").toString()), e.getMessage());
    }
    @Override
    public void onSuccess(Calendar cal, HttpHeaders responseHeaders) {
    log.debug("Calendrier créé pour " + cal.getSummary());
    }
};

for( String user : usernameList ) {
    cal = new Calendar().setSummary(user);
    this.service.calendars().insert(cal).queue(batch, callback);
}
batch.execute();