What Is A Shed 'Base Plate'?
My experience with a few sheds and the additional research performed today indicate that the base plate in the example provided is the structure in the first picture.

Requiring or recommending that anchors be secured to opposite diagonal corners would lead me to believe that the forces of nature applied to the base plate would be transferred through the 2x lumber framing, then to the 4x members and then to the anchors.

This requires that the attachments between the 2x lumber and 4x lumber be secure enough to handle those natural forces.

Without access to the 4x locations, as noted in the post, one would use opposite diagonal corners or perhaps all four, but provide for as much distribution of force as possible from the 2x lumber to the anchors.

If the inner portions of the frame are accessible, doubling up by adding another set of 2x lumber to be anchored would improve/increase the distribution of natural forces applied to the shed. This same logic applies to the four-corner option, rather than limiting yourself to two.

If one is particularly concerned about the forces of nature being applied to the shed, there appears to be sufficient room to add one each additional anchor and doubler to the longer sides.

Our professionally installed sheds secure the ground anchors at each end and each corner, 48" deep at the exposed 6x lumber, but strangely enough, into end grain, which I would not consider to be particularly strong (but it meets engineering design and local code!)