There's a difference between pathologically high complexity functions and Turing completeness. Sub-Turing languages generally don't allow recursion or unbounded loops - your program is always making progress. Solving 3-SAT doesn't sound like it precludes sub-Turing completeness, since you'd have a finite number of solutions you're iterating over.

It's still useful for a config language because it makes it harder to accidentally make a config that (in practice) never terminates, and usually allows for easier static analysis and refactoring of the config files through immutability and purity.