Earlier quoted context omitted.
If a monopolist is efficient, then nationalization is efficient. Break them up or seize them.
That does not follow. "The monopolist can do this efficiently" != "the government can do this efficiently". In particular, the monopolist has given evidence of knowing how to run that operation well. The government has given no such evidence. I'm not a fan of monopolies, but your position is flawed.
'Run well' like 'efficiently' is up for debate. Amazon is efficient because of its distribution model and allegedly because it exploits its workers to some degree. Many labor practices would be incompatible with standards of government employment, for the good reason that government should uphold the very labor standards it extols to the market. So if Amazon's retail distribution were nationalized, either it would run less efficiently or we'd have to admit that it's profitability depends on practices many consider unsavory or unethical.
But let's take ethics out of the equation and suppose you have a highly efficient operation run entirely by robots, which is also a monopoly. In a competitive environment margins should fall within the range of normal profits, but if it's a monopoly then the owner can continue to extract economic rents and you're essentially back to landlordism. In that case there isn't a good argument against nationalization or jacking the marginal tax rate sky high.