I mean, you could deus ex to-heck-with it and start the dungeon on a much lower level, but you don't even have to, based on the content as written.
I'll try not to go too spoiler-y here, but there are... methods... built in to the dungeon for inter-level transportation (and they do have protection against low-level characters accidentally using them to wander their instant death, so they may not be obvious).
For the sake of spoiler-friendlyness and simplicity of communication: I shall henceforth call them "gates", but that does not imply that they remotely resemble any sort of gate in the ways you think. Just know that there are... methods... to get to lower levels in the dungeon without traversing the entirety.
AIUI, they are there in part to avoid the otherwise inevitable "slog back through 10 levels too-low-to-be-worth-game-time critters" because you decide you need to head out to resupply / restock your tavern / head back to fraternize with your significant other / otherwise have downtime for a while and then come back in
There is such a "gate" fairly early on which leads to a level-appropriate layer, for a 10th-level party. If I were to run such a campaign, I might have them gain information from a third-party about the existence of this "gate" from a third party, before they even head in.
Perhaps they run into a quite seasoned adventurer at the Yawning Portal, suffering from PDSD (Post-dungeon-stress-disorder), who turned away from the life of a dungeoneer, and toward a life of crippling alcoholism.
A 10th+ level party may have some degree of renown, and perhaps having heard of them / recognizing their strength, the NPC speaks his tale of woe, losing his party in a horrific battle on a deep level, and being unable to recover their remains.
He asks them to help recover (whatever remains of) his fallen comrades to (return their possessions to their loved ones / get a res spell cast on them [true res is too expensive!]) and partially atone for his failure, and tips off the party on how to transport themselves to the lower levels to facilitate (or, even sobers up long enough to show them the way. Or, doesn't sober up, and still shows them the way - if the party is L12 in a L10 dungeon, having an escort mission with a drunk stumbling into all the traps, and making enough noise to wake a graveyard, and drawing out all of the denizens of the level at once could both level out the challenge and speed things along)