Lionel believes the dock is a Banksy because the writer fashioned the dock after [Flower Thrower][1] which is apparently what he believed was Banksy's most recognizable work since it is from 2003 and has made it's way across the internet by now.

Also, as you stated, it's a simple narrative device, and Lionel is a modified embodiment of [Clarke's first law][2], "When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong". 

He isn't elderly, but it's probably no coincidence that Lionel *is* a scientist as established by his position with Bron's company. In fact, the article I linked above goes on to say "His (Clarke's) definition of “elderly” was tongue-in-cheek: for those in physics, maths and astronautics."

Clarke's other laws make appearances in the film as well. 

the second: "The only way of discovering the limits of the possible is to venture a little way past them into the impossible." which a lot of the plot revolves around until the third act where the "magician" shows his hand to the audience (I won't spoil it for people who haven't seen the film).

And Clarke's (most famous) third law, "Any sufficiently advanced technology is indistinguishable from magic.” Which is also a constant theme in the film. 

I would wager the writer is classically trained and probably a fan of science fiction as much as he is of the mystery genre. Edit: [Rian Johnson][3], screenwriter / director of the film, graduated from the [USC School of Cinematic Arts][4] and before [Knives Out 2019][5], his prior 2 works are [Looper][6] and [Star Wars the Last Jedi][7] which are both in the science fiction genre.


  [1]: https://en.wikipedia.org/wiki/Flower_Thrower
  [2]: https://www.newscientist.com/definition/clarkes-three-laws/
  [3]: https://en.wikipedia.org/wiki/Rian_Johnson
  [4]: https://en.wikipedia.org/wiki/USC_School_of_Cinematic_Arts
  [5]: https://en.wikipedia.org/wiki/Knives_Out
  [6]: https://en.wikipedia.org/wiki/Looper_(film)
  [7]: https://en.wikipedia.org/wiki/Star_Wars:_The_Last_Jedi