Steps to reproduce:

1. mkdir temp && cd temp && npm init -y
2. npm i -D xo typescript
3. Create file a.ts:

export type T = {
  [key: string]: number;
};

4. npx xo
5. Confirm that the @typescript-eslint/consistent-indexed-object-style is reported as expected
6. Disable the rule from an overrides option in package.json:

{
  "xo": {
    "rules": {
      "no-await-in-loop": "off"
    },
    "overrides": {
      "files": "**/*.ts",
      "rules": {
        "@typescript-eslint/consistent-indexed-object-style": "off"
      }
    }
  }
}

7. npx xo
8. Observe that the error with @typescript-eslint/consistent-indexed-object-style is still reported.
9. As a sanity check move the "@typescript-eslint/consistent-indexed-object-style": "off" setting outside the "overrides" option:

{
  "xo": {
    "rules": {
      "no-await-in-loop": "off",
      "@typescript-eslint/consistent-indexed-object-style": "off"
    }
  }
}

... Now it works as is should... But why doesn't it work within the "overrides" option?