Assuming it has no imports or requires, and no references to meta, it’s a bare JavaScript file, and whether you treat it as esm or cjs shouldn’t matter, no?
Possibly the most devious nuance is whether the spec's appendix B applies, which affects whether html comment syntax is valid (yes, this is a thing). The html comment token can therefore be parsed either as a comment or as a series of operators depending on the grammar being used.
Effectively, this means it's possible to craft a program that does different things in CJS vs ESM mode.