Does UPDATE without WHERE clause lock a table in PostgreSQL?
An UPDATE without a WHERE clause will lock all rows in the table, but will not lock the table itself for DML. 

The rows can not be deleted from a different transaction because they are locked. 

But you can insert new rows without problems (assuming they do not violate any constraints).

Any row that is inserted after the UPDATE will not be seen by the UPDATE statement and thus they won't be changed.