Is spelling out the subtext a terrible mistake in writing? Is that related to "show and not tell" or is it a different thing? In the movie Predator, the subtext is that a woman who is underestimated is the biggest or can be the biggest threat to a man. And in the movie the main character literally spells it out to the audience. Is this a terrible mistake, bad writing or is it perfectly fine to do so? Why?
I haven't seen this movie so I can't comment on the specific example, but speaking in general ...
There are lots of things in writing that can be great if done well, and terrible if done poorly.
But in general, I'd say that spelling out the "subtext" ... which in this case looks like you mean the theme or "message" ... is generally a bad idea. If the audience gets the message from watching the movie, then spelling it out is superfluous. If the audience doesn't get it from watching the movie, then spelling it out for them is just going to emphasize that the movie failed to accomplish its objectives.
It's like explaining the punch line of a joke. Like saying, "You get it? Of course the chicken wanted to get to the other side! It's funny because you're explaining the obvious instead of really answering the question!" If the audience got the joke, they don't need this explanation, and the explanation will just be a boring waste of time. If the audience didn't get the joke, then explaining it to them isn't going to make it funny. They might eventually say, "Oh, I get it now." But it's too late to make it funny.
As I say, I haven't seen this movie, so it's possible that the explanation helps to reinforce or clarify the message. But I'd say, probably not. Even if it works in this case, I'd be very cautious about using it as an example.
Indeed, explaining a message like this is likely to be counter-productive. The audience knows that this is a work of fiction. The fact that you made a character who seems weak on the surface turn out to be strong doesn't prove that in the real world people like this character are strong. It just proves that you could write a story where you said that she was.
Of course anyone could do something like that in a story. If I wanted to spread the idea that, say, all Ruritanians are stupid and evil, I could write a story with several Ruritanian characters who are all stupid and evil. If I did it well, the audience might walk away thinking, Yeah, that's what Ruritanians are like. But if I have a character say outright, "You see, these Ruritanian people are all stupid and evil, and we should think of all Ruritanians as stupid and evil", now I've given away to the audience that I was trying to put a message into their heads rather than tell a believable story, and they're liable to start thinking, "Wait, are all Ruritanians really like that? Or did he just make the fictional characters in his fictional story like that? Which of course he can make the characters do anything he wants because he's just making up the story."