Manu Smriti as we have today is an interpolated scripture. However, the interpolation defense can not explain all the absurdities in Manu Smriti. So the best way to understand this kind of statement is by invoking Vivekananda.
The Smritis and the Puranas are productions of men of limited
intelligence and are full of fallacies, errors, the feelings of class
The basis of this is the upapurana named Brihaddharma, which allowed Brahmins of Bengal to eat fish. The english translation of the book is here.
Please also see the following from the book, History of Bengali people (page 364) by Ray.
Another of Bengal's scholars, did the same; quoting two verses from
the Vishnu Purana; he tried to show that, except for a ...
"My tongue never uttereth an untruth and my heart never inclineth to what is sinful. When my heart approveth of it, it can never be sinful.''
By these words he means that if a guy whose never lied and sinned in his entire life thinks its okay, then it actually is okay.
And it is true in today's world as well, if a guy who has never lied in his entire ...