There are superficial similarities, but important core differences. Bagelman is right that a Christian society would be socially very conservative by modern standards, but that is downstream from the real issue, which is that leftism does not have a concept of the Fall, and for that reason diverges from Christianity in some very important ways. Perhaps the most relevant divergence is the conviction of the leftist that the ideal society can be built on Earth by human hands, and objections to this project based in human nature (a concept that is foundational to the Christian anthropology, because that nature is now corrupted and therefore tends towards alienation from God) are typically dismissed as reactionary deflection. For the Christian, the ideal society can only be built by God, and I would go so far as to say that no properly Christian society can exist prior to the eschaton. With that in mind I would be inclined to agree with the topic quote, but in such a way that my agreement would be essentially worthless to the political leftist.
Catholics do have their social teaching however. And many European Governments have a tradition of Christian Democracy.
Christianity can and does have political ramifications, but it is not by its nature a political project. It is true that the Roman Catholic Church has social teaching, but never has that been understood as being its doctrinal core or the highest purpose of its calling.
Christian democracy is also not really a leftist political tradition, although it is an antifascist and more broadly antiautocratic one (hence the name) and seems leftist to many American observers because of its economic interventionism.
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