I think it’s reasonable to argue that the almost every democratic party has this principle.

Then the question is why multiple parties are necessary?

You can argue that they are wrong but that doesn’t mean they don’t believe they are following those principles just as faithfully.

We have concrete real world evidence backed by theory that this is in fact a fallacious idea.

If the single party’s ideology is so broad that it basically encompasses “don’t be evil” then I’m not sure I even understand the distinction between having one party and having a “partiless” state (which would effectively make factions within the party defacto parties in and of themselves).

The ideology, once again, is that the means of production should be publicly owned. This is not nearly as broad as what you wrote here.