Ah, misunderstanding on my part. For what it's worth, it doesn't say IP address, just listening address which I just took as whatever place (IP,port tuple) it listens on, and unless one has a huge IP range (uncommon with typical setups) I can see how "people" take that to mean port changing by default if they, like me, don't read carefully enough.
Huge ranges of IPv6 addresses are extremely common, virtually no ISP can be bothered to allocate smaller blocks than a /64. The minimum allocation of addresses recommended is a /32, which is 65k addresses.
You also know they don't mean port numbers because there's no such thing as a 6-digit port number.
From tfa:
"""Imagine your SSH server only listens on an IPv6 address, and where the last 6 digits are changing every 30 seconds"""
