It is not a plan. It was an idea - nothing more. Musk often expresses ideas for consideration. I personally feel that "nuking Mars" is the least feasible strategy.

"Humankind isn’t powerful enough yet to even consider changing a planet to our needs."

Make sure to consider self-replicating technologies - biological and mechanical. The right genetically engineered organisms could spread and change a great deal in a few decades.

Also, Mars does not need to be Earth-like to be livable. Humans can survive at one tenth sea level pressure. They would only need to carry a device to remove O2 from the atmosphere, which is mostly CO2.

People often forget that there are plenty of parts of the Earth were people need to "bundle up" to go outside. It is not that different.