I would still leave that up to interpretation. I think it can make sense however I am having trouble putting my interpretation into words.

Two alternative redemptions of the 'a' not being present: It could be viewed as an ellipsis, it does sound slightly more pleasant to me without.

And it would also just be really silly if the first statement said upon stepping onto a celestial body no human has set foot on before is a self contradictory statement is

But also everyone knows what he meant so it doesn’t really matter either way 🤷🏻