If you wanted to send a transaction you would need to pick a node to share it with. The goal of the Sybil attack is to try and increase the likelyhood that it gets shared to nodes the attacker owns. The attacker then shares the transaction directly to miners.
If gossip works by picking 3 random nodes to share transactions with if an attacker owns 1000 out of 1010 nodes they will very likely be the only one who transactions get shared to.
If gossip works by picking 3 random nodes to share transactions with if an attacker owns 1000 out of 1010 nodes they will very likely be the only one who transactions get shared to.