hyperloop was very low air pressure subway tubes for high speed trains. Works for research tesring tracks impractical to infeasible for real world applications. But it apparently what killed high speed rail in california (because wait this will be better)

The cars in tubes thing was a seperate stupid idea.

I don't know that he really wanted to fully execute on Hyperloop so much as build hype (and Tesla stock price) around the idea while sabotaging funding for California's high speed rail project. But yes, end goal to keep people buying his cars.

Average massively shitty car CEO idea

Actually not entirely true.

He was proposing transportation method similar to how the communication tubes are. Where vacuum would be used to transport capsules with people.

He knew it was bunk, so initially just published that idea and let anyone do it. His actual goal was to kill California's high speed rail, which he did.

Eventually he purchased Boring company and started building tunnel in Las Vegas and called that (as you called it, subway for cars) a hyper loop and pretend that it was the original idea.