That literally doesn’t make any sense. When Samsung announced it was going to 5 years it didn’t update all the stuff in the past. It’s a new thing starting with the new devices. They might give a little extension to devices still getting updates, but devices that already stopped getting updates are likely to have been upgraded from already and possibly harder to continue supporting.
Samsung was the leader. Google had to respond after years of being behind someone that doesn’t even work directly on the OS. Doesn’t stop Samsung being a leader if they change and go even farther.