Shouldn't it be the opposite?

[agecode-debug]

like
this.result.configFilename = isESMModule ? "eslint.config.js" : "eslint.config.mjs";
should instead be
this.result.configFilename = isESMModule ? "eslint.config.mjs" : "eslint.config.js";
No ?

[aladdin-add]

it's working as expected - the generated config is always esm, and it has be .mjs in type: "commonjs".

Have you encountered any problems? Please provide as much detail as possible.