They used AS55994 in China mainland.
In fact, China's ISP do filter via prefix and they all enable URPF. In China, IDC can't announce non-cnnic addresses
a) don't announce shit you don't own
b) know how to set up ACLs and prefix-list filters on your own egress IP space announcements which face towards your peers and IP transit upstreams.
Conversely, as a big ISP which has many small ASNs downstream of it, be responsible and set up filters on your own ingress which prevent your customers from announcing mistaken shit to you.
Using an example of a clueful and attentive major ISP: For example if you are a small to medium sized regional ISP and buy IP transit from NTT, one of the world's top-ten global commercial transit providers, they actually do take the time to verify each and every prefix you announce to them and will require an interaction with their NOC if you want to announce a new /22.