I'll answer it looking at existing reasons to delete the answer.
My personal opinion is that answers such as these do not add anything worthwhile to the site, hurt the sites reputation, and should be deleted.
Flag Reasons
I think regarding flagging, there are two options that can be considered:
very low quality This answer has severe formatting or content problems. This answer is unlikely to be salvageable through editing, and might need to be removed.
I'm not really sure how that flag is handled, but I doubt that it applies here.
rude or abusive A reasonable person would find this content inappropriate for respectful discourse.
The applicable policy would be:
Bigotry of any kind. Language likely to offend or alienate individuals or groups based on race, gender, sexual orientation, religion, etc. will not be tolerated. At all. (Those are just a few examples; when in doubt, just don't.)
In it's original format, I would probably have flagged the answer for this reason. The notion - based on antisemitic conspiracy websites - that "Zionists" funded Hitler's rise to power, as well as the linking of antisemitic conspiracy websites without providing context, seem to be able to likely alienate individuals based on the categories listed in the policy.
With the update, I think it's still a bad answer, but it's more difficult to argue that the answer violates the policy.
What else?
I think that a notice could have been added to the post, for example this one:
Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.
The notice specifically mentions reliable sources
, which antisemitic conspiracy websites are not.
In this case, the two sentences and their sources could also have been deleted, as they do not answer the question (the question is about Hitlers support of Zionism, not about "Zionists" support of Hitler).
And as always, downvoting is an option, which seems to have worked well in this case.
Conclusion
In this case, the sources could have simply been removed as off-topic.
In general, I think that a point can be made that holocaust denialism - depending on the exact context - may violate policy and should be deleted.
If this does not happen, a notice could be added and the answer could be downvoted.