Doesn't this proof not work when alpha or beta is 45°? 2 alpha would make the upper of the extension line in the diagram 90°, and alpha+beta for the bottom angl

Doesn't this proof not work when alpha or beta is 45°? 2 alpha would make the upper of the extension line in the diagram 90°, and alpha+beta for the bottom angle also 90°. The lines will never intersect, they're parallel