Could you provide an example where this piece of code breaks because of this assumption?

For example, when the JSON string passed is "OK"

Do you have a feature file that I can try out so I can see the example with some context?

Not at the moment. Anyway, you can simply have an endpoint returning "OK" and assertion that the JSON response is that string

For values other than arrays you could simply use Then the response body is: <PyStringNode> instead though:

https://behat-api-extension.readthedocs.io/en/latest/guide/verify-server-response.html#then-the-response-body-is-pystringnode

That would prevent me from validating that the content is a proper JSON string

You could probably do that quite easy with a custom check.

I don't personally see the benefit of fixing the existing code for this, but if you want to provide a PR I would most likely merge it.