If coaxial is half duplex, how come it's that fast? I have heard coaxial cables operates in half duplex. If thats the case, how come I get 500 mbps using a cable modem&router? Is the duplex just not that important? Or did I miss something?

"Fast" is very relative. The common DOCSIS variants support up to 10 Gbit/s downstream and 6 Gbit/s upstream in full duplex using _frequency-division duplex_ (different frequencies for upstream and downstream). Consumer-grade G.hn supports up to 2 Gbit/s.

Half-duplex with CSMA/CD was used with obsolete 10 Mbit/s Ethernet over coax (10BASE5 and 10BASE2). Ethernet has long since shifted to twisted-pair copper cabling (up to 40 Gbit/s) and fiber (up to 400 Gbit/s currently, 800 and 1600 Gbit/s under development), all full duplex.