But if you declare a string literal as const, then the string runtime value will match the string type. So it would be possible to have a runtime result (that executes an actual db query for example) that is paired with the type result.
No, TypeScript is compile time only. There's no runtime type info.
The parent comment is saying that if you write the string twice, you can get the compiler to check #1 and the runtime to use #2.
Not exactly (there is no duplication), a string constant in typescript is it's own type:
const sql = 'SELECT ...';
type SQL = typeof sql;
// type SQL = "SELECT ..."