Continuity mistake: After Becky charges the drone she climbs down without refitting the glass cover. Later a buzzard lands on the top of the tower, on the red cover.
Continuity mistake: After the ladder falls and before the girls realise Becky has been injured, the cut on her leg is visible in several shots. Sometimes her leggings under the cut are red with blood and sometimes they're not - it goes back and forth a few times. (00:41:35 - 00:42:55)
Continuity mistake: When Becky is holding onto Hunter as she hangs off from the top of the tower, the hand she holds onto changes between shots.
Continuity mistake: When Hunter is on the dish retrieving the backpack, during Hunter's and Becky's conversation, the rope switches from right in front of Becky to the next section over to the left of her and back frequently.
Continuity mistake: When the ladder falls off, several rungs remain at the top and are through the rest of the movie (eg. when Becky climbs down she attaches her rope to the lowest of these rungs). However, when the ladder first breaks and Becky is falling, there is a shot looking up at the platform that shows the entire ladder breaking off, with no rungs left attached to the platform. (00:38:44)
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.