But isn't it easier for AMD to close the gap due to their multiple dies on a package design instead of a large monolithic design like Intel? If they make the cores larger the cost for the entire CPU doesn't increase as much as Intels.

Even though Zen+ (cache optimization) is probably not as much of a improvement as Zen2 ("improves Zen in multiple dimensions") over Zen will be, the single threaded gap did shrink quite a bit:

https://i.imgur.com/dpfu5K3.png

But I guess some of these improvements come also from the faster memory that is supported. The benchmarks sadly were not made with the same memory.