I am hesitant to believe that both the existing monorail and an underground transit system are needed along the Strip. The monorail as built serves the casinos and other venues east of Las Vegas Blvd. , but it also presents a long walk to or from those places on the west side of the Strip. If they were to replace the monorail with a modern transit system under the Boulevard that would be helpful and fair to all businesses in the area. But there also had been plans to build a similar monorail on the west side, effectively replacing the several airport-like trams that now connect selected groups of casino/hotels. So if all that ends up being built out, there would be monorails on both sides of the street and an underground transit system as well. Railfan's dream, but more than likely impractical and overspending.
That said, I am not sure the writers of this article really meant light rail. We have underground light rail in cities like San Francisco and Philadelphia, but in both cases the underground lines also operate as streetcars on the surface outside of the downtown core. If such a line were built under Las Vegas Blvd., and remained underground for most of the way, I would venture to think that heavy rail might be a better fit than light rail.
Most importantly some form of rail transit must be built to the airport, while the ability to get between the Strip and the downtown area by rail must be possible as well if the intention of cutting down on vehicular traffic is to be realized. That would lend itself best to heavy rail under the Boulevard for the entire route. Short of that, major extensions will need to be made to the monorail, turning it into a system with hubs and spokes that connect at key points. The monorail so far has not shown itself to be profitable or useful to everyone, so perhaps it is time to re-evaluate things and look at the bigger picture rather than just connecting casinos on one side of the Strip.