Step-up can run further operations on the same operation context, which will be left with session metadata (lsid, txnNumber, etc.) from the last transaction the node aborted as part of step-up.
These operations can then invariant that the session is not checked out even though they have session metadata on their operation context. Ordinarily they would exit early for not having a stmtId here. That stmtId is assigned here from the update request, which comes from here. That would return an uninitialized stmtId, except the txnNumber is not null like it should be here.