How If commit statement can not ensure that dirty data is written to disk, how can data be read con？

Condition 1:

As we know, an important role of undo data is to ensure read consistency of the database: when the transaction of session A is not committed and session B wants to query the business data of session A's transaction subordinates, the query results come from the undo data.

Condition 2:

As we know, there is no mandatory correlation between whether the transaction is committed or not and DBWn writes dirty data from the buffer back to disk. 

For a non-commit transaction, dirty data may have been written to disk, but the corresponding data block is marked as uncommitted. Also, dirty data that has been committed may remain in the buffer.