[F]ederal law has been moving away from mens rea (“guilty mind”), a common-law tradition that suggests that a person who had no idea he was breaking a law should not be accused of doing so.
I'm no criminal law expert, but I'm pretty sure this is not the correct definition of mens rea. Mens rea doesn't ask if you knew you were breaking a law. It merely asks if you knew you were undertaking the activity that is legally proscribed.
To borrow from the example in the article, let's say it's a crime to "knowingly feed whales", and you threw some substance in the water and the whales gobbled it up. "Knowingly" is the mens rea term.* The article implies that if you didn't know it was a crime to feed the whales, you should go free. But that's not what mens rea means (indeed, it cuts against another common law tradition that has reached the point of cliche: "ignorance of the law is no excuse"). Rather, "knowingly" means that if you didn't know you were feeding whales (e.g., because you didn't realize whales were present, or you didn't realize whales would eat what you threw into the water), then you're not guilty.
* There are other mens rea standards, for example "intentionally", "recklessly", or "negligently". If there is no requirement of mens rea at all, then the crime is one of "strict liability" -- statutory rape is the most well-known example, where even an honest and reasonable belief that your sexual partner was above the age of consent will not serve as a defense.