There's numbers for Amazon logistics in the article.
More to the point, Amazon themselves use UPS/Fedex under the hood in addition to their own system. But they hold sellers responsible for using UPS/Fedex if it's late only when not using FBA.
> fewer than 25% arrived within two days; more than half arrived in about three days and more than 15% arrived in four days
Those numbers are for delivery speed. Not percentage of deliveries that are late relative to the date promised to customers when they made the purchase
Considering that it takes Amazon-level resources to get UPS/FedEx to track down errant shipments and honor claims, this seems like a fine idea at first glance.
More to the point, Amazon themselves use UPS/Fedex under the hood in addition to their own system. But they hold sellers responsible for using UPS/Fedex if it's late only when not using FBA.