CF, I read that article, which IMO is a clickbait headline. There is one quote from Speilberg and that quote is: "Is God our God only on this planet, or is God a God for every system where there's civilization?"
Additionally, I have seen the movie and I disagree that it's even a theme. It's a sentence that someone says, but they don't really explore those themes in any meaningful way, IMO. It is one of the failures of the movie. One character asks the question, another answers "no", basically.
I think it's an interesting question, but the movie doesn't put any work into addressing it.
You're really underselling this. Even Speilberg said it was a theme of the movie. It wasn't just some person with 1 throwaway line, it was a former nun talking with another nun. How much more obvious does it need to get? This isn't 2 random folks by the water cooler, this is 2 very religious people talking about the consequences to their belief system. It's a theme Spielberg wanted to address. Ok, maybe it's not the central theme of the movie, but it's there. The central theme is what consequences would this have and for people of faith the religious consequences are important.