Note that Microsoft has been using "custom chips" for years; they've just been on FPGAs not ASICs. They've developed IP to accelerate a whole bunch of processes, so it's not like they've suddenly come around to a magic sauce that Amazon and Google have been doing this whole time. It would surprise me if over half of the new chip is just based on RTL from their FPGA design.

The only thing that's changed is that they're scaling like crazy now and can justify overhead that comes with designing ASICs versus using off the shelf parts.