I'll take a try to explain in simpler terms. But first I assume you understand that Quorum Slices are to help a Stellar node do the validation.
Stellar is all about trust (validators). After all, when you have to trust somebody, you'd trust the reputated nodes, rather than a random stranger node on the internet.
You can imagine Quorum as a list of validators. Every Stellar node has its chosen validators in its own Quorum. For new nodes (e.g. by domestic users like me), we tend to choose the official/reputated nodes (e.g. nodes at SDF/IBM) to trust / to be the validators in our Quorum. You may imagine that these reputated nodes are like the media, newspaper or a TV channel. We ingest information from these generally trusted sources.
But it is not working the same the other way around. SDF and IBM nodes have their established Quorum. Logically, these reputated nodes only have other reputated nodes as validators, and it is rational to say they do not include "our domestic nodes" in their Quorum. Big brothers trust only other big brothers, rather than random guys on the internet like me.
Suppose we have this transaction: A sends B $100.
Sybil attacks are done by setting up many many nodes in view of taking over the majority vote. Yes, you may set up 1,000,000 nodes, and these many many fake nodes broadcast the false info "A sends B $44". But the reputated nodes do not have any of these 1,000,000 malicious nodes as validators, so the false info does not affect the big brothers. Also, for small domestic nodes, we depend mostly on the big brothers. Therefore, Sybil attacks do nothing to the non-malicious nodes.
Only 2 scenarios the network could go wrong:
(1) hack enough big brothers and make them broadcast the desired false information;
(2) many of the big brothers are colluding.
Yes, the big brothers are forming a small circle. But Stellar is all about trust. After all, when you have to trust somebody, you'd trust the big brothers, rather than a random stranger node on the internet.