Although it’s true that PK enforces NOT NULL, but if the keyword NOT NULL is not stated for the attribute, the NOT NULL constraint does not get created. Toad Data Model takes this into account.
Behavior of TDM concerning that NotNull is based on the fact that, in Oracle, there is a difference whether the tables is created by a statement with the attribute having NOT NULL constraint (In the database the NOT NULL constraint is created then.):
CREATE TABLE tabname (atr INTEGER NOT NULL PRIMARY KEY);
or the following way without the NOT NULL constraint stated (In the database the NOT NULL constraint is NOT created then.):
CREATE TABLE tabname (atr INTEGER PRIMARY KEY);
TDM considers the above difference during RE and in the first case the NOT NULL attribute is set, in the other it isn’t. It means that when generating the reverse-engineered database, the table gets created the same way as it was created originally.