There is nothing in the image that suggests that the bottom of both triangles forms a straight line.

Except for the part where it’s a single straight line segment, as depicted in the image. Showing the complimentary angles as an unlabeled approximately right angle is within convention. Showing a pair of line segments that do not form a straight line as a straight line is not.

Mathematician here; I second this as a valid answer. (It’s what I got as well.)