Tankies have read Marx and Engels, yes, but there are many other forms of leftism and even other forms of communism that aren’t ML. You are right about ML communists, in particular, but many other leftist movements are anti-authoritarian by their nature, so the point still stands.
Also, it’s possible to do the reading and disagree with the methods of implementation. I agree with the economics and the stated goals of communism, but I don’t believe authoritarianism is the best way to go about it.