It makes no sense for him to intentionally lie about something like that. It is widely known by anyone who plays Elite that Horizons is a paid expansion and it would make zero sense to try to pretend that players haven't paid for an expansion that FD was very obviously selling and continues to sell.
The only explanation that makes any sense is that Braben has been so disconnected from Elite development that he doesn't even remember what they were doing at the time Horizons was released. It's quite likely that he didn't really care where their revenue actually came from at the time because he was probably too busy trying to figure out how to move on to the next project, i.e., Planet Coaster, as quickly as possible so that he could make the company more profitable to investors. This wouldn't surprise me in the slightest given that basically every single video he's done about Elite has basically zero relation to the actual gameplay they've developed and nothing he talks about actually relates directly to the current state of the game. It's entirely plausible to me that shortly after the kickstarter he told the development team "hey I have some cool ideas" and they were only able to implement a fraction of those due to some combination of lack resources, disorganization or mismanagement and literally walked away from it after that point.
The idea that he would intentionally lie about having actually sold a product rather than providing it for free, however, is quite simply too ludicrous to even consider here.
No its not too ludicrous to consider. He's talking to Variety magazine and doesn't care at all about this forum. He's bragging about his company to mainstream media and he knows no one cares enough to go back and fact check and write another story about how some random video game publisher fibbed and didn't fully describe the cost of landing on airless moons in a four+ year old game.