It's not economy of scale - they get to write off depreciation at a much higher rate than the cars actually depreciate.

This is true of all businesses and all capital. It's a great big tax cheat, centered around companies getting to claim that their capital is worthless after just a few years.

How is this a “tax cheat” and not simply a tax deferral? Businesses can deduct expenses, and accelerated depreciation is not increased depreciation.

How would that actually help long term though? Imagine they write off their entire entire fleet in 1 year. Yes they pay less tax that year, but in future years when they sell their old cars that are worth nothing for lots of money they'll have a load of extra tax to pay.

I can see how it lets you move tax around in time a bit but that's it (and it's often allowed explicitly anyway).

Am I missing something or is this the common "tax write-off = magical money" misunderstanding?

On top of that the business referred to was a UK one and that means max 18%/year on ICE cars (6% if high CO2 emissions).