I believe to truly “get” the voice and texture of a given franchise, a tie-in writer needs to care about the world, characters and tropes of that IP; and that sounds like being a fan to me.
But the assumption of the people who usually say "Only a fan can write good X" is that you have to have a pre-existing fandom for the material, that being a fan to start with is the only way you can possibly know the subject or understand it, which is absurd, because it ignores the entire concept of learning. Yes, it's important to care about the material while you're writing it, but you can learn to love it. Bennett & Meyer weren't fans of Trek before they were hired to do the second movie, but they watched the whole series and found things in it that they loved and were glad to build on. I wasn't that fond of Enterprise in its original run, but when I was hired to take over the post-finale novels, I rewatched it twice and found things I valued in it.
Fans like to glorify and romanticize what they are, but really, learning to love media franchises is no different from learning to love any other subject you choose to write about. I researched the science of artificial space habitats for my novel Only Superhuman, read Gerard K. O'Neill's The High Frontier and the seminal research papers about space habitats from the '70s and the revisionist work that's been done since then, and I became a fan of space habitats, to the point that I now think colonizing and terraforming alien planets is silly and inefficient when you could build thousands of planets' worth of habitable surface area out of the asteroids and comets in our own Solar system.
So yes, you need to be invested in the material, but having already been a fan of it beforehand is not the only way to achieve that. A lot of the time, it comes naturally from throwing yourself into the research, immersing yourself in a subject to such a degree that it becomes intimately familiar and second nature.
And frankly, if you were not a fan of the IP, why would you even want to write for it in the first place?
As I mentioned, I was recently offered a shot to pitch for an IP I'd never even heard of, but it was a concept that was right in my wheelhouse, that aligned perfectly with the kind of things I'm interested in and with my skill set and focus as a writer. It's something I would've enjoyed writing novels for, even though I was unfamiliar with the media property the novels would've connected to. No IP exists in a vacuum; everything's part of a genre, of a wider style or category of storytelling, and you can be invested in the overall genre if not in the specific work.
Basically, I would've liked the gig because it would've let me tell the kind of stories I would've wanted to tell anyway in my original fiction. And to a large extent, that's what I've gotten out of Star Trek too. I wouldn't enjoy Trek writing if I'd had to follow the strictures of the '90s and just color within the existing lines of the franchise. I thrived as a Trek novelist because I had the freedom to fill in unexplored gaps in the timeline with concepts of my own, to explore the kind of ideas and themes I would've explored in my original science fiction if given the chance. The fact that it was in a universe I was already a fan of was nice too, but mainly I wanted to tell my own stories.
It's the same with Tangent Knights. While it's stylistically an homage to the tokusatsu shows I've been a fan of in recent years, the concepts and characters are largely recycled from old, unused comic book series premises I developed in the '90s. So the satisfaction is as much about finally getting to tell my own stories, in whatever form, as it is about paying tribute to a genre I'm a fan of.