I figured somebody might try to make this argument.
The problem with your comparison is that you're already starting from too much.
> You always start with a context
And where does that context come from? The true starting point is nothing. The context needs to arise from something. In order to even form a thought about apples and not any of the other thousands of concepts that you're aware of in the first place, you need a prompt.
In the case of an existing object, the prompt is seeing it. In the case of a non-existing object, the prompt is the combination of social and memory factors that I mentioned before. The former is simple, the latter is complex.
Another thing to consider: humans invented the number zero long after the natural numbers. Children learn to understand it after the natural numbers. The brain processes it differently, as mentioned by the article. There is overwhelming evidence that yes, zero is more complicated than the natural numbers for humans to think about.
If your reasoning ends with the conclusion that it's not actually more complicated, then that is in direct conflict with the evidence and it shows that you must be missing something.
> In the case of an existing object, the prompt is seeing it.
Not necessarily, not at all. It might be, "hmm, do I have any apples left in the sack?" Or "did my daughter retrieve the ears of corn"? Sometimes we randomly come across an object, but in a great many cases (the majority?) we already have the context -- we know what we expect to see (or not see), or what we're looking for, or what we're investigating. Humans are goal-oriented creatures; we're not just responding to current sensory stimuli.
> humans invented the number zero long after the natural numbers
Only in the highly technical sense of a dedicated symbol for balanced financial accounts, or a digit placeholder in a positional number system. Languages all have everyday linguistic equivalents for zero like "none", "no", "aren't any", etc. These mean "zero" in the counting sense, precisely and exactly. There's no evidence at all that these came after something like the number 7.
> and it shows that you must be missing something
Or you yourself, in this case. You are unfortunately looking at an overly limiting definition of context, an overly limiting definition of zero, and you're missing important parts of linguistic history.
> Using zero fundamentally requires more mental reasoning than using small positive integers.
I disagree. Positive integers don't exist in a vacuum either.
You always start with a context -- the thing you're counting -- and then you are given a number. Whether 3 or 0.
I see no greater conceptual complexity in either case. I think the flaw is in your sentence:
> This knowledge wouldn't be required if there was at least one apple.
Yes it is required. We're not comparing the statements "0 apples" and "1", we're comparing "0 apples" and "1 apple".