I personally don't believe this is evidence of the Mine Ride, for a few reasons.
First, to the best of my knowledge, no paperwork has been filed for that phase of the project.
Secondly, the shape and size of that excavation is very similar to that of a Disney waterway. The large green pipe sticking up is a storm drain, used for draining water to another location. Being in the bottom of the moat, where a drain would be, is logical.
There is an abundance of waterway being added as part of the FLE. It was originally to fall off Beast's mountain and become a large lake/river that flows under two bridges (one to the castle, one from FL back toward Mermaid) and into the originally planned meet-n-greet area. If you look at a photo of the original model, you can see the steep bank and waterway located near Cindy's cottage.
When they originally filed these plans with the county drainage board, the size and location of waterways essentially become etched in stone. Water storage and transportation is a huge deal in Florida, and it would have taken a significant amount of time to revise the plans and re-submit to the county in order to move the waterway.
Based on the perspective of the photo taken, that excavator is situated almost exactly where the river was to come down toward Cindy's cottage.