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.
Factual error: The events of the movie occur over 3 days, during which time a cellphone is used repeatedly, including for piloting a drone. There is no currently available high-end cellphone which would continue to operate this long without charging.
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.
Factual error: The drone used in the film would not be capable of the range depicted. For this small drone, using a usual 2.4GHz WiFi signal, a range of less than 1km would be expected. In addition, it is impossible to pilot a drone over 1km away without using hi-end optics to see it, as it is too small to see, let alone pilot. While there were binoculars on the platform, due to the plot twist, these were not able to be used.
Factual error: While the "life hack" of charging using a North American plug into a light socket might work, the use of a wedding ring to shorten the distance between the charger prongs and the fixtures, as depicted in the movie, would definitely not work, as there are two prongs that both need connection. All the wedding ring would do is to short out the connection.
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.
Revealing mistake: Becky calls Dan on her iPhone, as she puts it to her ear you can still see the contact info while Dan's voicemail is playing and not the actual call duration screen. (00:05:21)
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)
Other mistake: In an early shot that takes place at night, a woman in a restaurant points out that the radio tower has a blinking light. Later on during the night shots we see the strobe close up. The problem is, there is no electrical equipment on or nearby the base of the tower, nor are there electrical wires visible for the lighting. How is this tower still receiving power? Tower lighting is extensive, and there would at least be some form of equipment for it present at the bottom of the tower. Not to mention, tower lighting has to be maintained by FAA regulations, and it's difficult to believe a 2,000 foot tower would be in such bad shape. Considering the strobe at the top is incandescent, it would burn out often and the tower would need to be climbed.
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.