It’s more a promise than a guarantee but still a worthwhile fact to include in the value calculation.
600,000 miles is optimistic but even if you half that 300,000 is still quite a bit. The point being the battery deterioration is no worse and most likely far better than engine and transmission deterioration in gasoline cars, so the cost of battery replacement should not be factored into which one is better value.
Really, if it is not included in the warranty it is not worth mentioning.
The car is nice looking imo but it is so wildly over priced based on what it delivers for the money. Driving as most people do, say in the summer at highway speeds using the a/c you would be fortunate to get much over 125 miles on a charge.
The thing is simply not practical and that is why it offers so little real value.