Angular recognize imports from root?

I have a simple service which uses Inject/Injectable :

import {Inject, Injectable} from '@angular/core';
import {APP_CONFIG} from '../../config/app-config.module';
import {AppConfig} from '../models/core/app-config.model';

@Injectable()
export class AuthService {
    constructor(@Inject(APP_CONFIG) private config:AppConfig) {
    }
}

Nothing new here , however -as we know - I must import Inject, Injectable.

Even If I would've imported Inject, Injectable at the root module - still I had to import those in the file.

OK.

But now I'm facing a situation (code is not mine) where a developer imported some RXJS operators at the root folder :

rxjs-imports.ts

import 'rxjs/add/operator/filter'
import 'rxjs/add/operator/do'
import 'rxjs/add/operator/map'
import 'rxjs/add/operator/catch'
import 'rxjs/add/operator/shareReplay'
import 'rxjs/add/operator/distinctUntilChanged'
import 'rxjs/add/operator/pluck'

Then he imported the file in app.module.ts :

import './rxjs-imports';

And now he can use those operator without importing them(!) at another component/service :

 import {Observable} from 'rxjs/Observable';
//no imports for operators
  public get<T>(name: string) :Observable<T>{
        return this.subj.pluck( "d");     <---- how does it knows pluck ?
    }

Question:

I don't understand - how come it's working ? I was expected that the compiler will yell to add the pluck operator