In fact if you add all the numbers for the Alibaba and Yahoo Japan, it ads up to less than what it should be, i.e. on its own the core business has a negative value, which is kind of interesting.
No, Core Yahoo is a profitable multi-billion-dollar company in a slowly declining market.
The reason the market values Core Yahoo negative, is because it thinks the current management will destroy capital by spending it on useless acquisitions and costly R&D to become a growth company again.
But accepting that it is a stable company that's going to bring in billions in revenue for years, and just milking it dry is actually not a bad strategy. It's a very boring strategy though.