From the Account Activity Dashboard sample, it looks like this should include the webhook ID:
// request options
var request_options = {
url: 'https://api.twitter.com/1.1/account_activity/all/' + auth.twitter_webhook_environment + '/webhooks/' + req.body.webhook_id + '.json',
resolveWithFullResponse: true,
auth: {
'bearer': bearer_token
}
}
Looks like a documentation bug - will log this.