Isn’t this kind of understating the capabilities that have been implemented by @eric.snow for PEP 734? It implies that sharing has the same limitations as multiprocessing. Yes, currently sharing is limited at a basic level to certain primitive types, but those types can be shared without duplicating memory. Also, anything supporting the buffer protocol can be shared as memoryviews - and this has very low latency regardless of whether the buffer is very large!
(It also omits mention of the Project Verona team’s proposal at the language summit to add immutable conversion of objects and ability to share them across interpreters.)