They want to kill the Lich. They have a specific goal that needs to be done a specific way. If they have to kill the lich, then its railroading.
No specific object is needed to kill a creature. If the creature is immune to magic weapons in an all martial party, the weapons could be randomly generated weapons in loot.
It could also be Holy Water, Acid, Alchemist's Fire, or Poison.
If you lie to someone, you change their perspective on that situation purposefully to get them to do or think what you want them to without their informed consent.
If you don't give players information, they cannot consent properly to the quest. They'll go to the island and be disappointed and if you fail to impress them on the island, they'll feel like they were led astray and not given any real opportunity to leave.
Even if those spells are upcasted to 7th level?
What about a rogue with Investigation Expertise with a minimum roll of 18 Investigation that takes an action to discern him?
I don't consider it railroading if its part of the game (I still don't like the surprise! Nature of it). But when a player clearly should have succeeded when they failed to withhold information, that's railroading.