I guess you can call that smart. But it is also dishonest. They are effectively stealing from their employer. Many people seem fine with that, justifying it with the notion that their employer doesn't adquately utilize their skills, or is stupid to trust them so deserves to be taken advantage of, or didn't know how valuable their IP was so it was ok to take it and start a company around it.
I've known quite a few people who did this, even a couple who went on to found billion dollar companies while on the payroll and using company resources.