Personally I beleive all the bhagats like bhagat kabir and sheikh fareed and many others who arent mentioned in gurbani all go to Sach Khand. It's not a sikh only venue.
It's easy for ppl to manipulate a tuk of gurbani to sway it and justify their argument because gurbani is so deep and each tuk can have so many meanings. Whilst I dont have a shabad to hand I do think we need an open mind on this.
I heard someone mention an anecdote in a katha that 'Nanak Naam Jahaj Hai', gursikhs (if truly on the path I suppose) travel on the path of Nanak which is like a plane, other paths may be like travelling that distance on foot, on a bike, in a car with many more obstacles and much longer.
Not sure if this is the right way to see it.
Also if Bhai Khaniya could see God in all , even the enemy then we perhaps cant condemn others from reaching sach Khand for being on a different path.
Those whose actions are bad are unlikely to go to Sach Khand whether sikh or not.