Signal reflections inside CPU?
Most traces inside of dies are pretty short relative to the wavelength. An individual x86 processor core at 14nm is only 3x4 mm, while even the longer traces connecting things like L3 cache slices between cores in the Intel die photos look to be only about 4-5 mm long.  Compared to the wavelength (>40mm), that is pretty short.  My guess is they're not terminated, and they've picked that spacing and die orientation specifically to avoid it, but I could be wrong.

Conversely things like off die links (memory, PCIe, etc) are impedance matched because the distances are much longer.  AMD for example shows how they use on-die termination resistors for their chip to chip links in this diagram:

https://en.wikichip.org/wiki/File:amd_if-ifop-serdes.png