That seems unlikely to work out in their favor, if for no other reason than that car rentals turn a profit on top of depreciation and a ton of other overhead, and the cheapest rental cars are only a few times less expensive new than their Tesla.

As a ballpark estimate, each such trip would cost at least $50 after taxes and fees for a budget car rental (ignoring gas vs supercharging or any other fees that would be incurred in both situations, assuming there aren't one-way fees, etc). After 100 such trips you'd have paid $50k in rental fees, vs simply having a Tesla with an extra 100k miles. Resale value is a nonlinear function of a lot of variables, but linearizing a few portions of the parameter space for the current market an extra 100k miles would only devalue the car by at most $16k (less if they intended to hold on to it for a few extra years instead of selling it immediately while the mileage depreciation matters the most).

What they said was "a few times a year". Rounding up, that would be 25 years. In a few years, there will be plenty of electric rental cars and charger stations, and the batteries will be much better.

Right now, the most expensive wear in a Tesla is battery cycles.

The point wasn't that it works out in their favor if they make a ton of trips but not if they make a smaller number. Those figures were approximately a best-case cost for the rental and worst-case cost for mileage-induced depreciation at any point in the depreciation parameter space. If they only added 10k miles then they'd incur $5k or more in rental fees and $1.7k or less in mileage-induced depreciation by the above math.

I did misplace a zero though for what it's worth. A budget rental would only cost $500 or so over 10k miles, not $5k, if they're making the entire trip in a single day and returning it to the same place they picked it up. If that's the case the budget rental would probably be worth it financially (unless they were planning to hold the Tesla for awhile so that it hits a more favorable point in the depreciation curve), and if not then the extra rental fees are likely such that especially counting gas vs supercharging it's approximately a wash one way or the other.