I think it uses indexer and saturn (which ought to fall back to the DHT if it hasn’t cached the content of a CID already). That said they have a GitHub issue tracking CIDs that are retrievable via Kubo (fka go-ipfs) that aren’t yet retrievable via Lassie - it suggests to your point that there must be some difference; it also suggests they intend for retrievability parity though, so fingers crossed!!