Sybil resistance does not prohibit fake peers. Multiplying identities alone creates neither signatures from others' keys, valid coins nor work; it can still support routing, eclipse and surveillance attacks.
John Douceur described the Sybil attack in 2002: one entity impersonates multiple independent participants. Pseudonyms are easy to create, although obtaining suitable addresses or connections can cost resources. [John R. Douceur — The Sybil Attack]
Bitcoin has no global registry proving that one node corresponds to one person. A different IP address does not itself prove an independent operator. [John R. Douceur — The Sybil Attack] [Bitcoin Developer Guide — P2P Network]
A Full Node checks consensus rules independently. A majority of connected peers cannot vote an invalid transaction into validity. [Satoshi Nakamoto — Bitcoin whitepaper] [The Bitcoin Backbone Protocol]
Proof of Work ties weight in valid-chain selection to cumulative work. Advertising more identities does not by itself increase hashrate or chainwork. [Satoshi Nakamoto — Bitcoin whitepaper] [The Bitcoin Backbone Protocol]
Multiplying identities cannot forge another party's signature, create coins outside issuance rules or impose an invalid block. Checking these rules remains the Full Node's task. [Satoshi Nakamoto — Bitcoin whitepaper] [The Bitcoin Backbone Protocol]
Controlled peers can compete for space in a victim's address tables and connections. If they help capture its relevant connections, an eclipse attack can result; many identities alone do not prove complete isolation. [Bitcoin Developer Guide — P2P Network] [Heilman et al. — Eclipse Attacks on Bitcoin]
Diverse outbound connections, address bucketing and trial feeler connections in Bitcoin Core limit concentration. These are defenses, not proof that every peer is independent or a guarantee against isolation. [Bitcoin Core v29.0 — Peer connections] [Heilman et al. — Eclipse Attacks on Bitcoin]
IP addresses and Tor service addresses help route traffic; they are not consensus credentials. Multiple addresses can belong to one operator, while multiple users can share a network path. [Bitcoin Developer Guide — P2P Network] [Bitcoin Core v29.0 — Peer connections]
Proof of Work limits identity abuse in history selection, but does not solve every P2P abuse. Flooding, surveillance and manipulation of message availability require a separate threat model. [John R. Douceur — The Sybil Attack] [Heilman et al. — Eclipse Attacks on Bitcoin] [The Bitcoin Backbone Protocol]
A public counter of reachable nodes counts neither people nor votes on rules. Security is assessed through adversarial resources, network relationships and independent validation, not identity count alone. [John R. Douceur — The Sybil Attack] [Bitcoin Developer Guide — P2P Network] [The Bitcoin Backbone Protocol]
For the clearest picture, read this entry together with Nakamoto consensus, Eclipse attack, Selfish mining, Block propagation. The reverse links also lead from Byzantine Generals Problem, Eclipse attack.