AIUI Lassie uses indexers to locate content instead of using a distributed hash table (DHT). So a content identifier (CID) might be present on IPFS (and downloadable by a regular IPFS client), yet 'invisible' to Lassie.
If my understanding is correct, it can't be considered a complete IPFS client.
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!!
If my understanding is correct, it can't be considered a complete IPFS client.
https://docs.filecoin.io/storage-provider/architecture/netwo...
https://en.wikipedia.org/wiki/Kademlia