I would argue that FKR (and any other rules-light setup) requires a "shared understanding of the fiction" - something that well-written rules can provide instead. DnD doesn't require that we all 'get' how magic works in Toril. It simply tells us, in great detail.
I agree. But ... (in the words of that immortal Knight of the Round Table, Sir Mix-A-Lot, there's ALWAYS a big but ....)
There are a few ways of dealing with this.
First, I do think that it is harder to "play the world" if no one has any idea what the world is! In that, we are in complete agreement. For example, if you decide that the fiction you're going to run is Brideshead Revisited, and no one has read that book ... that's probably going to be an unrewarding experience. It will probably also be really difficult if only one person has ... I mean, the point of using FKR to emulate genre tropes (to emulate a world) does require at least some general understanding of the world. Or, put another way, do you know that friend you have that has no idea about Star Trek or science fiction?
Could they play Lasers & Feelings without any knowledge? Maybe? But it will probably won't be very fun for the.
However, we also have to remember that a lot of these games ... like D&D itself ... sprung forth without the prerequisite of knowledge! Arneson and his group were inventing that world on the fly, and that's okay.
Which gets to thrust of this- as long as there is a degree of trust between the participants, it's going to be okay. Really! I've ran enough of these to know that while it won't be perfect for everyone, it results in a rollicking good time for the people I play with.
Now, a slightly different issue is that of reliance- some people truly prefer having clear rules that are known in advance that they can rely on. And that's totally fine too! It's just a different way to approach a problem. As you correctly note, you don't have to have any shared understanding for magic in Toril- just rules. On the other hand, those rules also define the fiction of playing in Toril.