It wouldn't surprise me. They've been designing custom hardware for some time. Look at the Pluto switch and the "can we make something even more high performance" or "we can make it simpler, cheaper, more specialized and save some watts" (which in turn saves on power for computing and power for cooling costs).
At the scale that Google is at, it really wouldn't surprise me if they were working on their own silicon to solve the problems that exist at that scale.
Pluto is merchant silicon in a box, like all their other switches.
"""Regularly upgrading network fabrics with
the latest generation of commodity switch silicon allows
us to deliver exponential growth in bandwidth capacity
in a cost-effective manner."""
I wasn't intending to claim that Pluto is custom silicon but rather that Pluto is an example of Google looking for simplicity, more (compute) power, and less (electrical) power.
The next step in that set of goals for their data center would be custom silicon where merchant silicon doesn't provide the right combination.