How is this SLIGHTLY more?

[Allyrion]

I'm sure the Japanese language has words for "a lot more" or "a significant amount".

They understate things as a matter of form.
Clarity is not a priority in their phrasing as it is in English.

I'm not even here to defend the choice or anything. The idea is that he lied when what he said is quite normal when used in Japanese.
A little can mean a lot, but in less blunt terms.