I don't really consider it a challenge to read notes on a table that are clearly visible., Looking through a bookshelf to see what is on it isn't a challenge. Looking through a door and seeing obvious things on the other side isn't a challenge.
Even the rogue picking the lock. If they failed... so what? They have plenty of time to try again. Or, if you won't allow that, then the door could be destroyed by the Barbarian or the any of the casters.
You presented a room with doors, and yes there were actions the PCs could take to learn more information, but nothing that would require any sort of real challenge. So, how does this help demonstrate anything about challenging the players?
The first thing I need to do is give context. Mind, as much as he says this is a "whiteroom scenario," I built this actual dungeon with actual lore and actual structure.
Part of the challenge of exploration is, quite simply, the mystery. The cleric flipped through the notes and now a lich might be somehow involved. But is it behind the iron door? Behind the stone door is a dark room with an orb and a pedestal with corpses. Is it a trap? An ambush? Is the orb helpful or harmful? What was that clicking sound from the book? What did it do?
There are various ways to go about it, but the point of the exercise is that there isn't anything to beat in the core of exploration.
I don't think the exploration pillar is even supposed to be
about the challenge. Sure, you've got traps and obstacles and puzzles just like you've got monsters and terrain and spells in combat, but ultimately exploration, to its very definition, is the act of uncovering mysteries.
Its the experience of discovering something new. There is no "autowinning" exploration. In fact, doing something like teleporting past several dungeon rooms means you've probably auto-failed exploration because you've lost what could be valuable context.
Assuming a fair and honest DM running a fair and honest adventure, these contexts can be more powerful than the strongest magic items. You could convince a dragon to join your side if you know what he wants or you could trick the guardian into letting you past. You could stop a fight or engage in it.
Exploration is a pillar not because its identical to combat but because it shares its importance. A game with no exploration isn't just a game with no choices, its a game with no context. And once a game loses context, it becomes nothing more than just a string of mismatched talking and combat, which ultimately becomes boring for even the most mechanical players.