Opposing authoritarianism is implicitly saying they are not communist, saying they are against vanguardism (a word used by Lenin) means they are explicitly saying they are not communist.
Also as their web page says, they are not really a group with an ideology. They are a bunch of left-leaning techs who provide mailing lists, web site service etc. for left-leaning groups who "believe it is vital that essential communication infrastructure be controlled by movement organizations and not corporations".
My guess for what happened - the police were looking for one person or group that uses Riseup. As so many progressive organizations use Riseup, the targeting of one user or group of the service became a targeting of five locations, of people who provide the technological infrastructure of much of the left in Germany.
Just explicitly not Marxist-Leninist. I thought there were plenty of communists who made up with anarchists after balking at Lenin and the Comintern's demands in the 1920s.
https://riseup.net/en/about-us
Our purpose is to aid in the creation of a free society, a world with freedom from want and freedom of expression, a world without oppression or hierarchy, where power is shared equally. We do this by providing communication and computer resources to allies engaged in struggles against capitalism and other forms of oppression.
We oppose all forms of prejudice, authoritarianism, and vanguardism (possibly a reference to Leninist party vanguardism, which anarcho-communists reject.)
We work to create revolution and a free society in the here and now
Also FWIW, their favicon is a star in anarcho-communist colours.
To them, capitalism is a form of oppression.