You've made a circular definition. And I wasn't claiming that only nerve cells can form intelligence, but I think that if we expand the definition beyond what people normally call intelligent, then we should be more precise than the vague "ability to achieve goals in a wide range of environments" or else every complex adaptive system would be called intelligent, in which case we can drop that name because we already have one: complex adaptive systems (https://en.wikipedia.org/wiki/Complex_adaptive_system)

I wasn't to give a definition. Just refuting your claim that nobody would consider a tardigrade intelligent.

But if a tardigrade is intelligent then so are bacteria and even viruses. While you can define intelligence how you like, I don't think this coincides with common usage. In fact, you've extended the definition so much that it just means "life" or even a complex adaptive system (https://en.wikipedia.org/wiki/Complex_adaptive_system). If by intelligent you mean "alive" or "complex adaptive system", why use the word intelligent?

Also, if intelligence means "achieving goals in a wide range of environments", then I think some computer viruses are far more intelligent -- by this definition -- than even the most advanced machine-learning software to date.