The Chime of Opening is an example
For comparison, this is more specifically called out in the description of the Magic Item Daern's Instant Fortress (or Instant Fortress, if you're looking at the SRD) where it says...
It is immune to the knock spell and similar magic, such as that of a chime of opening.
The Chime of Opening does not actually cast the spell Knock. It does the following:
You can strike it as an action, pointing it at an object within 120 feet of you that can be opened, such as a door, lid, or lock. The chime issues a clear tone, and one lock or latch on the object opens unless the sound can't reach the object. If no locks or latches remain, the object itself opens.
Why it uses generic phrasing
This is often the practice of the writers of D&D content. They are future-proofing the campaign, and proofing it against homebrew. If you simply say "This door is immune to the Knock spell and the effect of a Chime of Opening" then that opens the door for players to argue that because their specific magical toy, which was released in a book written after this given adventure was written, is not called out as not working, then it should work.
By saying "It is immune to the knock spell and similar magic" then it covers everything that exists now, may exist in the future, or may be drummed up by a creative DM.