• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have read the FAQ and my problem is not listed.

Repro

module.exports = {
    "root": true,
    "parser": "@typescript-eslint/parser",
    "parserOptions": {
        "tsconfigRootDir": __dirname,
        "project": ["./tsconfig.json"]
    },
    "plugins": ["@typescript-eslint"],
    "rules": {
        "@typescript-eslint/strict-boolean-expressions": ["error"]
    }
}

function f(arg: "Option1" | "Option2" | null) {
    if (arg)
        console.log(`Input: ${arg}`);
    else
        console.log("Null was input");
}

tsconfig: the default as output by tsc --init

Expected Result

I expected there to be no error, as there is exactly one possible falsey value, no ambiguity.

Actual Result

  2:9  error  Unexpected nullable string value in conditional. Please handle the nullish/empty cases explicitly  @typescript-eslint/strict-boolean-expressions

Additional Info
This is similar to the likely non-bug #2894, but in that example there are multiple falsey values.

Versions