"Defensive systems" can be interpreted broadly to include cybersecurity.

But yes, it's an arms race. Saying it's not an arms race isn't going to make it not an arms race. Warning that it is an arms race isn't ethically wrong.

Is participating in an arms race ethically wrong? Maybe you could ask the Ukrainians how they feel about drone R&D?

Getting out of an arms race is harder than it looks, but there's at least talk about "pacing" and that's a start.