Yeah it's not they way I have done it before either. But the clearances where so tight, the semi-symmetric design, and the with the number axis i had hit just made sense to try something new. Plus with 3d scans of the motors and chassis available it was a logical choice. The cost of laser cutting metal has dropped too, I can probably get the whole frame and motor mounts for about 800$. That is really cheap compared to water jet or CNC. I have seen alot of RC guys do similar construction techniques for drones and planes in wood, GRF, and carbon. One turned me on to this (attached) paper on slot and tab design and strength analysis for metal construction and it all kind of clicked. Plus, if anyone else wants a subframe, or wants to tweak the design for use in something else, copies are very easy. Where as if I one off-ed a subframe as I have done before, making another is almost the same amount of work.
These slots are all 0.2 mm tolerances on the tabs, and it makes for a really tight fit without glue. It is near the limits of the laser cutter for accuracy, especially and the curves. And with riv-nuts and other interesting fasteners, in not sure you absolutely even need to machine the parts at all anymore. Just cut, weld (or glue!), add your hardware, and install.
But this is kind of what I am doing this project for. It really is a prototype for techniques and ideas I have been playing with for some time. I could have just built a by the book (what ever that is) SLC, but I hope to push the envelope here. Wait to you see the battery design....nothing like it in the automotive EV space yet.