I agree with you. Singular "...is a lie..." sounds better to me in English. (Incidentally, it also seems to be closer to the wording originally used by Bakunin: the Russian text of Statism and Anarchy as given on this site uses the singular noun "ложь": "Всеобщее и поголовное право избирательства целым народом так называемых народных представителей и правителей государства -- вот последнее слово марксистов, так же как и демократической школы, -- ложь, за которою кроется деспотизм управляющего меньшинства, тем более опасная, что она является как выражение мнимой народной воли." I don't know what version of the text Marx was reading, although this blog post indicates that he read a Russian version, not just a German translation.)
Some English translations of this section of Statism and Anarchy do in fact use the singular: the one available online at the Marxists Internet Archive (which gives its source as Bakunin on Anarchy, translated and edited by Sam Dolgoff, 1971), uses the wording "This is a lie...".
As user341275 mentioned, "lies" might have been used because the previous sentence mentions two groups, "Marxists" and "the democratic school", but I disagree that this makes it a good idea to use the plural "lies". If two groups are both promoting the same falsehood, it remains one lie, not two. The "as well as" in the first sentence makes the use of "They are lies..." at the start of the second understandable, but I don't find it very acceptable in comparison to "This is a lie...".