Decentralized mix-and-forward designs remove the server, but repeated local mixing can attenuate global information under heterogeneity and exposes peer-to-peer neighborhoods as a privacy attack surface. To preserve FedAvgstyle aggregation semantics (over updates reconstructable by the round deadline) while scaling dissemination, we present FLTorrent, a BitTorrent-based dissemination layer for serverless FL with a short warm-up. Warm-up hardens within-round source unlinkability—a dissemination-layer goal orthogonal to content protections (e.g., DP or secure aggregation)—via (i) pre-round obfuscation, (ii) randomized lags, and (iii) coordination-only non-owner-first scheduling (tracker off the data path), before switching to vanilla BitTorrent swarming. We upper-bound the per-transfer attribution posterior by the fraction of owner chunks in a sender's eligible cover set, and derive a tighter highprobability bound that improves with early non-owner mass. A simple heuristic, GreedyFastestFirst, attains ≈92% of a bandwidth-optimal max-flow upper bound, while warm-up remains a stable ≈12% share of a round across 100–500 peers. Under an observation-only local adversary, FLTorrent drives attribution success close to neighborhood-level random guessing for typical nodes, improves with network size, and remains robust under collusion. In LLM-scale dissemination stress tests over 7–10 Gbps access links, FLTorrent adds only ∼6−10% round-time overhead relative to BitTorrent-only. Overall, FLTorrent shows that within-round unlinkability and BitTorrent-level efficiency can co-exist with predictable, low overheads at scale.