ESlint Angular - no-unused-vars hits type definition

I'm upgrading/refactoring an Angular project (to Angular 8, Electron 6, Ionic 4) and we decided to switch from TSLint to ESLint.

I setup some rules and they are running but I can't get a rid off the no-unused-vars warning for type definition. When I run linting I'll get this warning for OperatorFunction and Observable which is obviously not an issue.

import { OperatorFunction, Observable, timer } from 'rxjs';
import { tap } from 'rxjs/operators';

export function executeDelayed<T>(fn: () => void, delayTime: number): OperatorFunction<T, T> {
return function executeDelayedOperation(source: Observable<T>): Observable<T> {
//...
}
}


.eslintrc.js file uses this configuration

module.exports = {
"env": {
"browser": true,
"es6": true,
"node": true
},
"extends": [
"eslint:recommended",
"plugintypescript-eslint/eslint-recommended",
"pluginrettier/recommended"
],
"globals": {
"Atomics": "readonly",
"SharedArrayBuffer": "readonly"
},
"parser": "@typescript-eslint/parser",
"parserOptions": {
"ecmaVersion": 2018,
"sourceType": "module"
},
"plugins": [
"@typescript-eslint"
],
"rules": {
"no-unused-vars": [
"warn",
{
"vars": "local",
"ignoreSiblings": true,
"args": "after-used",
"argsIgnorePattern": "res|next|^err"
}
],
"no-use-before-define": [
"error",
{
"functions": true,
"classes": true
}
]
}
};


I went trough multiple similar questions here but wasn't able to find the solution. Any idea? Switching back to TSLint is NOT an option.

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