High Sierra was released before the 9th generation Intel CPUs became available, so it is normal the CPU is "unknown". It will work properly.
My main system uses a 9th generation Intel i7 to run MacOS Sierra 10.12.6 / High Sierra 10.13.6 / Mojave 10.14.6 / Catalina 10.15.7. In Sierra and High Sierra the CPU was labelled "unknown". As it seems to work properly I just ignore it.