That's not what I mean though. What if I have a cluster with devices I don't trust, but I want to let them emit logs if they conform to a particular protocol. Like, will this thing check signatures for me and such?
Since it doesn't say anything about trustlessness, I assume that it assumes that all nodes are trusted.
LogDevice is crash fault tolerant not byzantine fault tolerant if that's what you're asking. This fault tolerance is in regards to where the logs are placed not who's emitting them though. If you want to analyse logs for inconsistencies or attack patterns you should look into something like SEAMS/REAMS, it's completely out of scope for LogDevice.
LogDevice is payload-agnostic and doesn't inspect the value of the binary blobs it stores. If your writer is allowed to write according to ACLs, LogDevice will happily take writes from it regardless of their content (upto the max payload size). Verification of payload content should be done on a layer above LogDevice - either before taking the write or when reading.