Yeah, they are different but...
The window frame is almost exactly the same, just larger on the 8 series.
They use similar tricks of upper and lower body lines to visually lengthen the shape which makes them look shorter.
The both have a lower, lower body line that helps slim down their mid-sections.
The BMW uses the wraparound taillight to finish off the hip line, while the Mustang uses a small, separated extension of the upper body line to continue into the tail.
Made this gif to help me decide if they are too close.
After seeing them together, they are different enough, I guess.