IMO this isn't a 100% accurate description of what the claims for AI risk being hard are. (And I would generally be against using inaccurate claims to attract people to legitimate areas)
For starters, arms race is probably a better terminology than externalities. When I think "externalities" I think - this technology creates X amount of private good and Y amount of public harm, and selfish actors have incentives create them. And while this description can be forcibly fit for AI x-risk, it is important to remember that x-risk is a harm from a selfish perspective too. Even if you don't care about the survival of other people, you care about your own survival, and therefore AI that causes your death or else permanently disempowers you is a harm to you too. An actor who honestly believes their AI has high probability of causing x-risk will not unilaterally deploy it anyway. Unlike, say, a fossil fuel company that will produce fossil fuel fully knowing the CO2 footprint they are creating, or a cigarette manufacturer that will produce cigarettes while having accurate statistics of the expected number of future cancer patients they are helping create.
In an arms race however, it can make sense for someone to pursue technologies that have significant x-risk, if there is also (in this case, utopian) upside they wish to capture before an opponent does.
Externalities is also not a good framing because it assumes that state regulation is sufficient to solve the problem, and prevent private actors from consuming public goods. It assumes the problem to be easy assuming govt support, and the hard part is primarily one of public protest and lobbying the government. However it is currently uncertain how much of a role state regulation will play in AI risk. It forces the problem into a conflict-theoretic frame rather an epistemic disagreement. Both kinds of problems require very different solutions.
However, arms race is also not the best terminology. For instance, Hayden Belfied has a post that cautions against prematurely claiming that US and China or OpenAI and Deepmind or whoever are in active arms race towards AGI. Link to post. Most progress iscurrently being pursued by unilateral actors who are (somewhat) open to reason and willing to signal in favour of motivations besides profit or military superiority. OpenAI for instance has explicitly ensured a non-profit stucture to govern them, for this reason.
Arms race is also not the best terminology because many AI x-risk researchers believe that AI x-risk is high even if you take the arms race out of the equation. Even if humanity agreed on one coordinated effort to deploy AGI, and did it slowly over a period of say a few decades, there are researchers who claim AI x-risk will be hard to mitigate, and that AI systems will be default be very hard to control. Yudkowksy (and a few others) go as far as saying we will only get one real shot at the problem in practice, in which we will risk extinction.
The core claims of AI x-risk are legitimately weird and I don't think it is easy to make it non-weird. Given this to be the case, I would caution against diluting the claims and prefer debating them head-on instead.
It's risky to connect AI safety to one side of an ideological conflict.
There are ways to frame AI safety as (partly) an externality problem without getting mired in a broader ideological conflict.
I think you can stress the "ideological" implications of externalities to lefty audiences while having a more neutral tone with more centrist or conservative audiences. The idea that externalities exist and require intervention is not IMO super ideologically charged.