When things get extreme they get similar.

'Extreme' is a vague word, but when you're talking about communism and fascism (or more generally 'far-left' and 'far-right' ideology), that's a false generalization known as 'horseshoe theory'.

There are many clear counter-examples when talking about communism, like the entire school of anacho-communist ideologies and the existing societies stemming from them (including the Zapatista territory in Mexico with a population of around 360,000, or the FEJUVE federation in Bolivia, or the many anarchist communes around the world).

As for the more authoritarian versions (Stalinist, Maoist and related ideologies), despite their strong one-party systems, they are still extremely different to fascist ideologies in their goals and how they use their strong state to achieve them. To say 'they are the same in many respects' would apply just as equally to liberal capitalist states like the USA and allies, with their infamously militarized police, constant wars and imperial militarism, strong cult of nationalism (for the US, it's centered on the Founding Fathers), mass imprisonment and state interference in bodily autonomy.