Summary: ASICs are built by taking existing designs and disabling but not removing stuff because designing new circuits is extremely expensive and error prone. They don't have the money to build an ASIC yet. They also do not trust existing designs. An FPGA both is allows them to iterate towards a final design easier (and with user validation by using it) as well as take advantage of software and cryptography to secure the final SoC design which runs on the FPGA.
Summary: ASICs are built by taking existing designs and disabling but not removing stuff because designing new circuits is extremely expensive and error prone. They don't have the money to build an ASIC yet. They also do not trust existing designs. An FPGA both is allows them to iterate towards a final design easier (and with user validation by using it) as well as take advantage of software and cryptography to secure the final SoC design which runs on the FPGA.