All my traffic is routed through a VPN. When you connect to a VPN node, every user on that server shares the exact same public IPv6 exit addres that’s just standard VPN routing. The 'ISP shifts' you're seeing are actually the VPN's hosting data centers . Because most VPN apps automatically connect to the fastest/optimal server, users in the same region get routed through the exact same sequence of servers. The matching IPs and network patterns are simply how the VPN functions, not proof that we're the same person. There are few addresses because I'm using the free version.
I believe most of us know how VPNs work, and you are correct in your technical explanation.
The point isn't just that they share the same IPv6 address. When theymos says you "switch ISPs following the same patterns," it indicates that they may be analyzing the history of both accounts and finding a pattern of coincidences, repeated over time.
A coincidence can be explained by the VPN, but a repeated pattern of coincidences becomes more difficult to justify in that way alone.
Furthermore, we must take into account that many other Turkish users (and not only) may need to use a VPN to access the forum. Many probably also use the free options. If this pattern were common among these users, it would be difficult for theymos to highlight something, as you did.
Either way, I'm only speaking based on the information that has been presented.