Anyone who believes in the validity of any other religion other than Islam (perennialism) and dies believing that, has died as a kafir and is bound for hell forever.
Sikhism and Hinduism both believe that all religions are paths to salvation.
This is also found in the Tariqa Notes of Nuh Keller, but he has been quoting Hosein Nasr in his translation called "Reliance of the Traveller", despite knowing that Hosein Nasr is a perennialist.
but i still cannot understand how keller calls ling and co. Muslims without the proper tahqiq?
But these people might be notorious in equivocating, and basic their own taqiyya to avoid detection.