... yes.

If it was 16K lines of modular "compositional" code, or a DSL that compiles in some provably-correct way, that would make me confident. A single file with 16K lines of -- let's be honest -- unsafe procedural spaghetti makes me much less confident.

Compiler code tends to work "surprisingly well" because it's beaten to death by millions of developers throwing random stuff at it, so bugs tend to be ironed out relatively quickly, unless you go off the beaten path... then it rapidly turns out to be a mess of spiky brambles.

The Rust development team for example found a series of LLVM optimiser bugs related to (no)aliasing, because C/C++ didn't use that attribute much, but Rust can aggressively utilise it.

I would be much more impressed by 16K lines of provably correct transformations with associated Lean proofs (or something), and/or something based on EGG: https://egraphs-good.github.io/

On the other end of the optimizer size spectrum, a surprising place to find a DSL is LuaJIT’s “FOLD” stage: https://github.com/LuaJIT/LuaJIT/blob/v2.1/src/lj_opt_fold.c (it’s just pattern matching, more or less, that the DSL compiler distills down to a perfect hash).