Since he also has two other vehicles, he was clearly using those to drive too. He's sold them now, but he doesn't mention anything about the cost of Turo which he suggests as an alternative.
I have no idea exactly what Uber breaks down to per mile, but ridester.com suggests $1 to $2.50 per mile. Let's assume it's as cheap as $1/mile to give him the benefit of the doubt and account for UberPool.
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At $1/mile, he traveled 12k miles over 6 years - or 2000 miles/year.
* Maintenance on 12k miles would basically be nothing. No major work should need to be done in a 12k window. Probably some wipers, some fluid changes, possibly a minor repair - let's just say it's $500.
* Gas - Let's assume 20 miles/galloon and $4 per gallon. $2400 over 6 years.
* Insurance - The most expensive part. 2000 miles/year is nothing, but insurance doesn't work that way. Let's assume a modest $500/year. - $3000
When you add that up it comes to roughly $6k left over. You can look at this two ways.
1. Try to buy a vehicle in the $6k range and assume no-resale value. You're not going to get a great car, but it will be doable.
2. Account for only depreciation. 12k is mostly negligible miles. The biggest lost will be due to aging. That being said, as long as you avoid relatively late-model year, you can purchase basically any mid-upper car and still break even (like an M3).
Plus, in both cases you get the benefit of actually owning a vehicle and not relying on 3rd parties.
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EDIT: He also mentions using Turo as an alternative for a car. That opens up the possibility of him renting out an own vehicle on Turo. An M3 would likely catch $100/day or more. Let's say $50/rental day gets to his pocket. If he rents a out his M3 just twice/month (e.g. one weekend) and assuming the driver hits the 100 mile/day standard. He'd add only 14.4k more miles (still not a lot) and make an additional $7.2k. That pays for depreciation and leaves him room to ride share while his car is gone.