Where exactly is this stated? As far as I can tell, all this does is say what taxes you should be collecting, and then collect them. It doesn't even handle (at this point) remitting payment to the appropriate jurisdications.
I see no evidence that if your taxes are collected wrong, Stripe will do anything other than say "Your accountant should have caught that, sorry." I see no evidence that by using this product, Stripe will indemnify you against tax issues (which yes, would go a long way to justifying the cost)
I am no Stripe pricing authority, and have no relation to the company, but I'm pretty sure that is *not* what she said.
She said that Checkout does not have an additional cost on top of transactions fees, not that Tax does not have an additional cost.