Is there a guideline on Lemmy why we let the tankies be part of this

It depends on your instance. There are 3 main tankie instances. Hexbear, Lemmygrad and lemmy.ml my instance blocks the first two but allows the last. This is because ML users are generally not obnoxious and keep their politics within their instances while the other two run wild all over other instances. Personally I dont feel like ML users are that bad. There is right wing lemmy instances but Majority of the instances de federated from them over their disgusting views. I think lemmy does a good job and has the infrastructure in place to promote a healthy range of political views.