“Beneath the Monolith” was also put out by the same company that produced the original setting with its bespoke system (Numenera/Cypher System). They just know which way the wind blows and strive to maximize their profits.
Multiclassing is an optional rule in D&D5e, not allowing it should not be controversial.
This is done in movies and books to give the reader/watcher a chance to learn the setting without getting hamfisted about it, stuff is naturally explained to the character because they don’t know it either. That’s not necessary in TTRPGs, since the player can always ask out of character.
Also, it’s a way to get some character into the story the reader/watcher can identify with. This also is not necessary in TTRPGs, since there the players naturally identify with their character (at least in most games, some do that differently).