If we really want to be so pedantic then the term unlimited can never be used in this context due to the Bekenstein bound.

IMO the term is still a useful marketing short-hand that has come to mean "more than can reasonably expected unless you are purposefully trying to break the system", though cellular data providers in particular have done their part to cloud the issue by having "unlimited" terms that are really ridiculously limited.

Based on my dealings with Amazon so far I have every reason to believe their "unlimited" will be, if anything, overly generous.