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Amazon also do similar with their ENA [0]. James Hamilton talked about it in 2016 [1].

The youtube link at the 35:00 mark says that the networking is an custom ASIC not an FPGA. "Amazon Annapurna ASIC" is what the slide says.

Are the ENA instances using something else?

First gen ENA (never publicly acknowledged AFAIK) was an FPGA, second gen is an ASIC.