Have you actually read Newton's thesis on ancient cubits?

Newton's theory was based on a reported example of the temple cubit from Jerusalem which he labelled as an authentic example, as mentioned in a previous post.

Newton never suggested that the temple cubit was a feature of the design of the Great Pyramid.

Newton supposed that the base side-length of the Great Pyramid was 400 cubits as 20 times the length of the King's Chamber with the latter equal to 20 cubits. This theory required an approximate length of 688 English feet,and was based on the survey report of Greaves.

This particular survey error by Greaves had no bearing on Newton's rationale on the length of the temple cubit which was based solely on the so-called authentic example.

Newton was, however, badly misled by Greaves in another matter. You may spot this by looking at the drawing of the cross-section of the Grand Gallery in Newton's dissertation.

Mark