Why check for multiple nonces despite finding a golden nonce?
There's a difference between what is called a "golden nonce" in the context of the source and a valid block header hash.

To reduce payout variance, miners group into pools and share their income proportionally to their hashpower. 

To determine each miner's hashpower, pools use a share system: when you are mining and find a block header of high difficulty (with lots of heading zeroes) but not high enough to be above the current target, you send it to the pool. These difficult-but-not-enough headers are called "shares".

Since finding them is way more frequent than blocks (miners often send several of them per second), no matter how small your hashpower is, it will still be taken in account at payout time even if you're unlucky. The more shares you send, the more hashpower you have, the bigger the share of the pool's income you will get.

They are not broadcast to the bitcoin network, since they would be immediately rejected there and are of no value to anyone but the miner that found them and the pool software.