What you’re referring to as “Leninism” is in fact Marxism-Leninism or as I’ve recently taken to calling it, Marxism plus Leninism. Leninism is not an ideology unto itself, but rather another component of Marxist ideology developed by Marx, Engels, and others comprising scientific socialism as defined by Marx and Engels.
Where do you see that? It says Marxism-Leninism, so they are subtracting Leninism from Marxism
Much like you can write 0÷0, you still cannot solve it. Leninism has a base in Marxism, it is therefore already a part of its development and cannot be understood without it.