not quite. One of the things like like about the top mount approach is that you
need repeatability to achieve accuracy, but not accuracy.
You are drilling three holes that matter, and then cutting around them. As long
as the machine will always drill the holes in the same place, you can move the
workpiece under it to cut the different parts. This includes cutting the
mounting holes in the sled.
Even if the sled ends up a drastic oval, the resulting machine will be accurate.
The 45 approach can also cut out the arms with the same method, but not the
holes in the sled (although there’s nothing magic about 45 degrees, so you could
cut three holes, put a pin in the middle, cut around the outside, rotate it to
approximately 90 degrees from the first cut, and cut the other holes)