The PCs are supposed to level up throughout the Citadel
They might not be able to open the door at level 1 if they find it at the earliest possible state - but they can open the door once they cleared some other parts of the Citadel. Basically they can open it on their way back out if they want to do it.
Using this as a hook if your PCs ignore it or don't have the required abilities (yet)
You can of course also use this door as a little hook for getting the PCs to return at a later time, especially if they choose not to check the door on their way out or simply forget about the special door they couldn't open in the beginning. This is in line with the adventure providing you with
two entrances to the Underdark - one on the upper level and one on the lower level. These are explicitly not fleshed out so that you may use them at a later time if you choose to do so.
How you do this is up to you and your imagination. For example you might choose to:
hide these entrances in some way, or block them and use a special quest item found in the parts that follow the door you mentioned that acts as a special magical key. Or you could let NPCs tell your players some lore about what awaits them past this door. These are just two quick examples - the adventure provides you with opportunities to make this a location your PCs want to revisit at a later time if that's in line with what you want for your campaign. Imagine you wouldn't have anyone in your party who could turn undead or use the Knock spell on level 2 - they wouldn't have a chance to open the door. But maybe an NPC is willing to help them in exchange for some loot and in need of an escort. Or some noble is willing to pay handsomely for information and evidence of what lies in these corners of the citadel because of some old story his father and his grandfather told him when he was a child. Endless possibilities...