Camera zoom works by enlarging the center portion of the image, or in other words, by making its field-of-view narrower. Zooming in on the setting sun will not reveal more of the sun, and will only enlarge the size of the sun in the resulting image.
Flat-Earthers are claiming that zooming in on a setting sun will reveal the entire sun, and somehow lift it out of the water. They are wrong. They simply used the incorrect exposure settings. In reality, zooming on the setting sun will never reveal the sun that is already obstructed by Earth’s curvature.
Usually, they conclude such a thing due to the wrong exposure settings being used. They used autoexposure and let the camera decide the exposure for them.
When fully zoomed-in, the sun appears larger and fills more of the frame. As a result, the frame is dominated by a bright object (the sun), and the autoexposure system will compensate by lowering the exposure. The sun will appear in the correct exposure with a clearly defined edge. This is done by sacrificing the appearance of all the other objects, which now appear dark. The camera —rightly so— does not think the other objects —like the sky and the ocean— the objects intended by the photographer.
On the other hand, when fully zoomed-out, the sun appears very small and occupies far less of the frame. The picture is dominated by dark objects, like the evening sky and the ocean. The autoexposure system now thinks the photographer wants to take a picture of the entire landscape and compensates that by raising the exposure. As a result, the sky and the ocean appear in the correct exposure, but at the expense of the sun, which now looks too bright, or overexposed. Not only that, the glare around the sun will appear too bright as well, and it can be impossible to distinguish the sun from its glare. Flat-Earthers think the glare is part of the sun, and incorrectly assume the sun is larger than it is.
The autoexposure system can only guess the intention of the photographer. Sometimes it guesses wrong, like when the intended object is far brighter than its surroundings, or vice versa. To take a proper picture of the sun during sunset, it is necessary to use manual exposure so that the camera does not give us the correct exposure to take a picture of the landscape, but not for the sun.
When using manual exposure to correctly expose for the sun, it will be very easy to confirm that zooming-in will never somehow bring the sun that has already set back into view.