I have seen people who call themselves libertarians openly arguing that it should be legal for companies to sell poison in food—I don't believe baby food came up specifically, but that's a purely emotional addition, and shouldn't change the overall argument—because the free market would work it out.

You may not think they are credible, or libertarians, but I'll ask you not to deny my lived experience just to support your personal idea of what a libertarian is or should be.

I've heard some Democrats argue for some pretty awful things. I dismiss them as not credible representatives of the DNC.

Nobody thinks it's good for poisoned food to be sold.

The libertarian argument that you're almost certainly mischaracterizing there is about the pandora's box that is opened by allowing the government to decide what is or is not "poison" (which is of course an absurdly emotionally charged word picked by their opponents). Consider that some governments want to restrict sugary food on health grounds. Sugar isn't a poison but once you accept the principle that governments can ban companies from selling "poisoned" food, the definition of poison immediately comes under attack and sooner or later means almost nothing like what you'd expect.

That is a good point, but I was specifically talking about putting unambiguously deadly poison in baby food.

There are always going to be grey areas, such as the line between "safe" and "unsafe", and Libertarianism doesn't provide any guidance on where that line should be. The government's role is to define that line, and the courts to interpret it as part of checks and balances.