I have very little worry in the short term--or even in the medium term (next several years) about an Apple car seriously competing with Tesla. As others have pointed out, no battery manufacturing plant, no automotive manufacturing plant, no charging infrastructure, etc.
Without a battery plant, they either buy from existing battery suppliers (which means they won't be able to compete with Tesla's price using packs from the Gigafactory), or their production volume would have to be so small that they wouldn't seriously threaten market share.
Without an automotive plant, they have some real amount of time they'd need to ramp up before they could produce even a subset of Tesla's current production rate. Granted, they have tons of cash, but cash doesn't build or automate a car factory overnight.
Without fast charging infrastructure, their car is nothing more than a city car, which seriously limits its appeal to the car buying public. (Chevy Bolt, anyone?). And even with tons of cash on hand, you can't just build the equivalent of Tesla's Supercharger network overnight.
I just don't think this Apple car, if it turns out to be real, will be competing in Tesla's market space. As a result I don't see it truly threatening Telsa's stock price for years to come, if ever.