A rather questionable statement, but that is what it says. Of course harmony always depends on the style - Bach´s harmonies are rather different from Ellington´s.

In classical harmony you have to be careful with using the seventh when you are making a second voice - they are lovely when used right, but they have to be prepared and to resolve according to the classical habit, otherwise it just doesn´t sound classical anymore.

I guess this comes from a jazz context, where it is more or less true ("Don´t be afraid of playing wrong notes: there aren´t any!").

I would phrase it as above for the specific context of learning to develop a second voice