The Sun emits sunrays to every direction. But as the Sun is very far, the sun rays that reach us are practically parallel. To an observer on Earth, the rays of light coming from the Sun form a maximum angle of about 0.53°. They are practically parallel, but not perfectly parallel.
Flat-Earthers often question the fact we say sun rays are parallel, but in any diagram of an eclipse, they are drawn at an angle. They merely confuse practicality with perfection. Sunrays are practically parallel, but they are not perfectly parallel.
In most practical purposes, we can afford to simplify and assume that sunrays are parallel. For example, with a sundial, we don’t need to take such a minuscule angle into account because the gnomon is not far from the base, and won’t affect the function of the sundial.
But in the case of an eclipse, we are dealing with considerable objects in addition to the Sun itself: the Earth and the Moon. Both are separated by quite a distance, about 30 times the diameter of the Earth. Thus, the angle of sun rays is critical to the problem, and we have no choice but to take it into account. In the case of eclipses, we need the angle to determine the type of solar eclipse (total or annular), the path of totality, the length of a total lunar eclipse, etc.
In short: sunrays are practically parallel, but not perfectly parallel. Flat-Earthers are often pedantic. When conversing with them, it can help to add the adverb ‘practically’ in front of every adjective. Otherwise, they might unconsciously add the word ‘perfectly’ in front of every adjective themselves, and use their own pedantry against us.