They didn't. They had been saying for years that it would require a computer upgrade that could be done with a service visit, and that the service visit would be free for people who purchased the "FSD" option.
They absolutely did, and then changed the story when they announced they were building HW3.
> All Tesla Cars Being Produced Now Have Full Self-Driving Hardware
> Oct 19, 2016
> To make sense of all of this data, a new onboard computer with more than 40 times the computing power of the previous generation runs the new Tesla-developed neural net for vision, sonar and radar processing software
^ this refers to the nvidia thing 'onepremise says was too bad for the job.
I don't have time to dig it all up, but they were talking about the potential for computer upgrades as far back as 2016 as well: https://twitter.com/elonmusk/status/789008557341454336 (though not for pre-2016 vehicles).
I can see where people who don't follow the company closely wouldn't have been aware of this, though. And, obviously there have been plenty of false and misleading statements made in this area.