Yup, you were right. When I put those two triangles on there, using the exact angles of 7:1 and 5:2 triangles, the angle between the vertical line at the right side of Giza and the horizontal line at the bottom was 89.9319255°. The vertical line's azimuth was 0.4288579° West of due North and the horizontal line's was 0.3603698° S of due W. That was after I used the sides of the big regular triangle as the hypotenuses of both triangles.

When I tried it by starting with the two right triangles from the centers of the two pyramids, with the horizontal side of the 7:1 triangle set to the same azimuth as the S side of G1 (0.0613557° S of W) and the vertical side of the 5:2 triangle at 90 degrees to that one then the big triangle between their hypotenuses had an angle at the hill point of 60.0684881°, at G1 center 59.6326654° and at G3 center 60.2990141°. So the most accurate 60° is at the hill point. The other two are less accurate so maybe they were just trying for the 60° at the hill point more than the other two. If G3 was positioned after G1 was built then they obviously couldn't stand at the center of G1 to survey the lines out, but they could stand at the hilltop and lot lines visually to the centers of both pyramids.