Trivia: For the film, a real radio tower was built, albeit significantly shorter. The film was shot on a tall mountain, to give the illusion that the tower was much taller. A blue screen was placed below it. The two main female characters did their own stunts.
Trivia: Profanity Warning: The original cut of the film contained over 30 uses of the word "fuck." When the film was picked up for theatrical distribution, the studio wanted to redub the film's profanity to make it PG-13. The studio had the actors redub their lines with less harsh language, and then used A.I.-based software (similar to "deepfakes") to digitally manipulate their mouths to match the new dialogue.
Answer: There's no indication that the ending is anything but real; the other twist in the film is heavily foreshadowed through dialogue, camera framing, and so on, none of which applies to the ending. A helicopter could have hovered above the tower and lowered a rescue basket/rope to Becky, as she's less than 100 feet from the very top. Given that the tower was probably constructed using a helicopter to position the final pieces of the structure (a common practice for communications towers of this height), there's no reason why it couldn't have held up. The tower itself is never shown to be weakened or compromised, only the ladder affixed to the outside.