There’s nothing about Disroot’s politics on the website.
You’re right in that the website isn’t hitting you with anything overly political (though the picture on the homepage doesn’t look like “we’re just a normal email service”). It is clearly a project by activists. The TOS say you can’t be “Contributing to the discrimination, harassment or harm against any individual or group. That includes the spread of hate and bigotry through racism, ethnophobia, antisemitism, sexism, homophobia and other forms of discriminatory behavior.” Now that doesn’t sound too bad and I guess GMail and most mainstream providers will tell you similar things in their TOS but “contributing to” is so unclear that it could mean they block your account if you subscribe to Brian Lunduke’s newsletter or something. That’s probably not what’s happening, but still it would give me the feeling of a sword of Damocles hanging over me. That being said, I like what I read on their about page.
Interesting find! Apparently you can get access to their mail service by participating in their forum. “From 4 May 2023, verifying your identity via a code sent to your phone will be replaced by a reputation system that grants access to Vivaldi Webmail only to users who actively take part in the community.”