I'm not sure why you are comparing them on a threads per socket basis.
Each Xeon Phi core has 4 threads which results in 288 threads per socket. [1]
At those core counts you are often no longer limited by the CPU. Memory bandwidth starts playing a more significant role which is why the Xeon Phi also has 16GB of high bandwidth memory with more than 400GB/s. Does the Power9 system use a similar memory technology?
Power9 is cool, but its really not comparable. Power9 has a huge L3 cache, 10MB every 2 cores. So a 22-core machine would have 110 MB of L3 cache.
In contrast, Xeon Phi is connected to HBM2 / Stacked Memory. So while it doesn't have as large of an L3 cache, its main-memory is significantly faster.
Finally, you can stick a Xeon Phi into each PCIe x16 slot of your servers. Really, Xeon Phi is a competitor to NVidia's Tesla V100, if anything.
Each Xeon Phi core has 4 threads which results in 288 threads per socket. [1]
At those core counts you are often no longer limited by the CPU. Memory bandwidth starts playing a more significant role which is why the Xeon Phi also has 16GB of high bandwidth memory with more than 400GB/s. Does the Power9 system use a similar memory technology?
[1] https://en.wikipedia.org/wiki/Xeon_Phi#Knights_Mill