if I'm able to ping 8.8.8.8 in 10ms, doesn't that mean that my rtt latency to get out of the docsis-part of the network is 10ms, and that any latency beyond that will be the same regardless of docsis/fiber?

Yes, the majority of that latency is likely coming from DOCSIS. You're likely in a major city, large enough that Google has an anycast DNS server nearby.