How does it differ in reasonableness from "but the one who constructed all things is Supergnome" ?
When most people think of the use of the word "god", they think of someone or something, that is
more powerful than humans... and deserving of worship... in some cases.
For example, celebrities are treated like gods, because people think they have abilities
beyond normal.
The original Hebrew word 'El' means 'god', which probably means
'Mighty One'; 'Strong One'.
So people for centuries understood one(s) mightier than living things on earth, to be god(s).
See
Gods.
Almost everyone is familiar with Superman, so this may be easy to understand.
The Secret Hebrew Meaning Behind Superman’s Real Name
Superman was written by two Jewish writers, and his birth name Kal-El can actually be translated into Hebrew, revealing a secret meaning.
The name Kal-El has two parts, and each can be translated into Hebrew. Kal, or קל, means "easy" or "light" - but it's important to remember that with the creators' accents at the time, Kal would be read as Kol, or קול, meaning "voice." El in Hebrew, אל, means "God," thus Superman's name in Hebrew is קול-אל, which loosely translates to "the voice of God." This does not necessarily mean that Superman is God incarnate or even directly sent by God, but rather is an instrument of God - a person with incredible powers who saves lives and fights for the weak and downtrodden.
I found this article quite interesting, so I encourage you to read it. It's quite short.
So hopefully you got the point.
Anything
mightier than normal / natural physical abilities is associated with 'El' - 'god'.
It's not a made up idea, that is used to fill some gap, but it is an understanding that ancient people have had long ago.
Think of it this way...
Suppose scientists did discover ETs - a type 2 civilization, when you tell children who are new to this world, ETs-type2 built X, you explain that on the basis of what you know. Not what you made up.
Likewise, Those writers are explaining what they know, rather than something they made up.
So that's the difference. God isn't made up. Supergnome is.
The fact that it requires someone to build a far less simple structure, like a house. It requires
a far mightier someone, to build a much more intricate structure, like the earth, with its water cycle; nitrogen cycle, etc. That someone is known as God.
Why someone?
The house requires planning, a goal, and instructions.
This is what we observe in the things around us. For example, the earth, with all its resources necessary for earth dwellers.
That is using logic, reason, thinking ability, intelligence. It's being rational.
Imagining that chemicals came together on their own - undirected, random, etc. to form all the matter in the universe would be like imagining Supergnome, I think.