What is the name for this sarcasm-like form of words?

Question

Is there a technical name for the common form of words whereby we refer to a thing -- often a person -- having a low level of some property, by saying that it does not have an especially high level of that property?

For example:

John is not the smartest person in the room.

I recently used this form myself (which is what triggered this question), in an ell.se answer I gave, when I wanted to refer to someone being of low intelligence. I referred to them as having:

...less than Einsteinian levels of intelligence

In practice, this structure is often combined with an idiom, as in:

• Billy is not the smartest saw in the shed.
• She was no oil painting.
• Money doesn't grow on trees.
• This is not my first rodeo

(But just to be clear, I'm asking not about the idioms themselves, but about the general form where "not lots of X" is used to mean "very little X")

Prior to asking here, I searched around a bit and so I think I can eliminate a few possibles. It is not:

• sarcasm, because the statement being made is true;
• damning with faint praise, because the statement doesn't actually praise at all;
• a backhanded compliment, because it is not, in fact, a compliment.

So what is it called?

Answer 1

It's a form of litotes:

a figure of speech and form of verbal irony in which understatement is used to emphasize a point by stating a negative to further affirm a positive

Source: Wikipedia. The article lists an example which is not unlike* your first one:

Along the same lines, litotes can be used as a euphemism to diminish the harshness of an observation; "He isn't the cleanest person I know" could be used as a means of indicating that someone is a messy person.

It's an old rhetoric device, already used by the Ancient Greeks and Romans (hence the use of a Greek/Latin term for it).

*_{: see what I did there?}