You are correct. Lightweight transactions perform a read-before-write [1]. The read phase is performed with a serial consistency which requires a quorum of nodes in the local DC (LOCAL_SERIAL) or across the whole cluster ( SERIAL) [2].
Quorum of 2 nodes is 2 nodes so RF=2 cannot tolerate a node outage. Cheers! [1] https://www.datastax.com/blog/2019/04/lightweight-transactions-datastax-enterprise [2] https://docs.datastax.com/en/cassandra-oss/3.x/cassandra/dml/dmlConfigConsistency.html#dmlConfigConsistency__table-write-consistency
