As you stated, it's a simple narrative device, Lionel is a modified embodiment of Clarke's first law, "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, screenwriter / director of the film, graduated from the USC School of Cinematic Arts and before Knives Out 2019, his prior 2 works are Looper and Star Wars the Last Jedi which are both in the science fiction genre.