But this should not use the subjunctive, because he has indeed drawn an image using the scale of the Moon being one pixel wide. This shows it's possible, and hence the subjunctive is inappropriate.

That's not really correct. Even if we assume that this is no longer a hypothetical/counterfactual case, then the correct grammar is to use the present tense ("This is the moon as one pixel"). But, in this case, it still is counterfactual. This is really short for "If the moon were only [the size of] a single pixel", a statement which we know is not correct.

You're probably confusing this with the rule surrounding verifiable facts (ie, "I asked John if he was happy").

Quoting from the Wikipedia page:

    Subjunctive forms of verbs are typically used to
    express various states of unreality such as wish,
    emotion, possibility, judgment, opinion, necessity,
    or action that has not yet occurred.

The action has occurred, it is not an unreality. It seems to me that using the subjunctive would be inappropriate.

And yes, perhaps using the present tense would be more appropriate. Certainly I would have used something like:

    The solar system plotted on a scale where the Moon is one pixel.

My comment still stands - the subjunctive is inappropriate. Saying it should be still something else does not make that less true.

<Shrug /> It's the interwebs, people will write what they like, and declare that nothing is right, and nothing is wrong, it's all OK. In that case telling someone to use the subjunctive seems doubly inappropriate.

https://xkcd.com/386/

English is not my native language, but using "was" sounds pretty weird to me. In latin languages, when a sentence starts with "if" and the verb is in a past tense form, it must be the subjunctive form of the past tense. I remember in primary school they teach the past of the subjunctive with "if" before all the pronouns. Don't know how they teach the past of the subjunctive in English though.