First, it took Tesla a total of 15 months from taking over their factory space until they released the Model S. Not a big deal, since they had been working on the design for a few years prior, just as Apple has been doing.
Contrary to the mythology, there is nothing in manufacturing cars that is some sort of voodoo magic known only to those with a century-long history in building cars. All the component suppliers are well known--glass, tires, brakes, electronics, steel, plastic, tooling, robotics, molding, wiring, control software, batteries, transmissions, etc, and Apple has tight control on their supply chains as a matter of policy. Experienced engineers are available and apparently already on the payroll in quantity.
Plus battery-powered cars are much simpler than internal combustion cars. The biggest issue will be battery supplies, and that is absolutely a logjam in the making. Everything else is nuts and bolts, and has been done a thousand times before all over the world.
When you think there are significant barriers to entry, you're looking at it all wrong. Apple has no vestigial, 1960's facilities, work force, locations, tooling, methodology, etc. Nothing they have to work around or hold onto. The barrier is simply some time and a pile of money, both of which they have in abundance.
As far as slim margins, ask Ford and Chevy how much they make on those pickups. And that's putting them together fairly old school.