At https://github.com/filecoin-project/lotus/issues/6111 it's revealed that infura does not support all operations and is used via:

export FULLNODE_API_INFO="https://PROJECT_ID:PROJECT_SECRET@filecoin.infura.io"

It also mentions:

export FULLNODE_API_INFO=PROJECT_ID:PROJECT_SECRET:/ip4/filecoin.infura.io