The question might be mixing apples and oranges.
Worldbuilding is locations, cultures, history, politics...
The elements you need the reader to "buy" is not the general scenery, costumes, and language details of every exotic location – that's just worldbuilding. This is what Sanderson describes as an iceberg that is partly visible but mostly inferred.
"All the monks in this town wear blue stockings."
who grows the largest cabbage becomes king for a year."
mountain people speak a different dialect than the river people."
Some of these are deliberately quirky or exotic. Maybe they will be worked into the plot for wonder or conflict, but for the most part this type of worldbuilding is about setting which is largely going to be dictated by the genre.
The caution against over-worldbuilding with this sort of background detail is that it can't possibly all be important to the plot, or work as a conflict to the characters. Worldbuilding disease is when these descriptions become infodumps because there is no subtle way to integrate all these details. At some point the scenery and culture is competing against the story and characters for "screen time" (or reader attention).
Stories which support extensive genre worldbuilding, often use simplified characters: hero archetypes, villains with uncomplicated motives, stock enemies and townsfolk, and familiar story structure like the Hero's Journey.
The "buy" is something so major it should be world-breaking.
Sanderson does not describe this very well, but he warns against "something ridiculous appearing on page 400". His examples in the video (around 26:00) is that Jedi is a funny word or the killer's mask is made of plastic – these are not buys (imho), but he emphasizes the problem is that the buy is so major that it will knock the reader out of their suspension of disbelief if it hasn't been supported by the narrative.
Writing Excuses discuss the 'buy" with more depth, so rather than say it is a "rule to be broken" it would be better to listen to the topic podcast How Weird is too Weird. They begin similar to Sanderson, saying if a buy is introduced too late in the story it can be rejected by the reader. The example is a story where dragons are the buy, and suddenly a sword you've had all along is magical – this feels a lot like deus ex machina, it's world-breaking without a story that builds to it.
Later they describe the buy in terms of supporting too many world-breaking elements:
"Women have bugs for heads" AND "gravity doesn't work the way you
think it does".
"A POV warrior in a sci-fi battle turns out to be a giant 6-legged ferret, which isn't mentioned until much later in the novel."
They also suggest that the number of buys the reader will accept depends on the length of the story (1 buy for TV episodes and short stories, maybe more in longer stories), and genre – including a "new weird" genre.
Again the emphasis is that the reader will accept 1 big buy as the premise, but too many become arbitrary and the reader has no terra firma on which to stand. The problem is not too many competing elements or overloading the reader with details, but knocking them out of the story by abusing their suspension of disbelief.
Worldbuild-y details are fine (in moderation), but the reader will only accept a limited number of major world-breaking concepts in your premise.