TL;DR: No, the Nix community shall not accept any military companies working on immediately violent projects.
It’s never easy to exactly figure out where to draw the line between slightly acceptable and slightly unacceptable, but that is no excuse when it comes to excluding obvious cases.
NixCon was and still is a community conference. A conference of various people¹ interested and involved around the wider Nix ecosystem. (Human) Beings from different backgrounds coming together due to their shared interests.
This means that the design of such a conference should be focused on the event being welcoming to all these human beings first and foremost.
Others in this thread have already outlined that the offensive presence of military actors can be off-putting to people at different levels – or not. There are former military filks like @jonringer who feel positive about the presence of military actors. Others might not care at all in any way. Then there are folks who feel uncomfortable because this conflicts with their ethics or politics.
But those which a caring community needs to consider explicitly are marginalised people who might not just be made uncomfortable, but are affected so strongly that they do not feel safe at the conference and thus cannot take part. People like those described by @shanesveller.
In the specific case of Anduril, it’s not just the military part that might ostracise certain marginalised groups, there are also Palmer Lucky’s right-wing ties that can make the event even more hostile. [that’s a different can of worms though]
It is unlikely that folks like Jon won’t contribute to NixCon when military corporations are banned as sponsors. But it is likely that larger parts of the community won’t come when such sponsors are permitted.
Because sponsorship of such an event might be partly rooted in the honest desire to give back, but there are of course additional perks of being a sponsor.
Having your company logo all around normalises your presence, frames all visitors of the conf to be somehow part of the same community as you.
Also, buying into such a conference inserts a sponsor at a central interaction hub visitors cannot really avoid. As an individual community contributor you have a more fine-grained choice in which projects to be involved in and with whom to interact. When visiting a conf, you cannot avoid having to deal with their sponsors.
At NixCon23, Anduril employees where still among the visitors, one of them even held a talk. But there, he was only responsible for exactly this work he presented. By being a sponsor in contrast, you make the full conference in-scope of your name.
¹Yes, I know the idea of corporations being people in the US-sphere, but even there the people representig that corporation are still humans.