You just reminded me of a very funny memory of mine. This material in this chassis is definitely hardened, because you can't really machine unhardened aluminum. Imagine trying to cut a piece of freshly chewed bubble gum. That's what unhardened aluminum cuts like.
I only know this because we got this huge 5000 lb piece of 6061 that was supposedly T6 hardened, but they sent us an unhardened piece by mistake. My buddy spent 6 hours setting it up on the machine and I was standing there when he took his first cut, which was a doosey. He fired up the vertical lathe at a blistering RPM. Imagine a 5000 lb part, about 5' tall, and 5' diameter, spinning so fast you can't make out writing in black sharpee letters about 6" tall. It would actually be a gray blur. He started the tool 1/2" into the part above it. As the tool fed down to cut the outside of the part, aluminum goo instantly formed onto the tool and he ended up with a basketball sized blob of aluminum stuck to it before he could get the machine stopped



One of the funniest things I ever saw in a machine shop. First and only time I ever saw unhardened aluminum in a machine shop.
These parts are likely 6061 T6 (or somewhere close to T6 anyway), which will machine, but being 6061, it will still bend fairly easily.