I will assume the operative part of the shabad relating to the issue of dietary bibek is they anoint their kitchens trying to become pure. First one must consider the entire context, and not simply refer to a line here or there for understanding. It's talking about inner corruption such as plundering people and ignoring weeping children while attempting to engage in purifying practices.
If one doesn't wash one's hands it introduces bacteria into food. So it is a health practice to keep your hands clean when touching and preparing food. Also, the kind of diet you eat has a profound effect on the health of the body, and in subtle ways on states of mind by affecting brain chemistry and hormonal balance influencing mood. If you are a vegetarian, you don't want meat contaminating your food. We've come to a point in society today where all kinds of garbage contaminate the food. You know about the mad cow disease epidemic? Well, cows are vegetarian, yet it was common practice to put in commercial feed rotting carcasses of dead beef, so the cows were ingesting dead cows, and thats what got them sick and spread the disease. Why would society be so unclean, so insane in its practices? What tolerance encouraged this mentality?
First of all discernment is a spiritual practice that allows a person to use common sense. Bibek strictly prohibits meat-eating so ingesting of diseased and contaminated food like this would be avoided. Check labels of foods to avoid secondary contamination of things like gelatin or rennet which contain ground fish bones, pigs hoofs and cow's stomachs found in things like cheese and yoghurt. If you want to talk about actual arrogance, consider science which develops all these weird and ghastly permutations of food products in order to cater to business industries that care nothing for the disease promoting qualities of the food they contaminate.
It's true almost everything in the world is impure, and our attempts at spiritual purification are futile. Guruji gives us the Naam, through which the whole body-mind-atma are purified. Bibek is first about about avoiding jhooth. Since the atma is encased in physical form, the nature of body-mind have a direct effect on one's spiritual jeevan. Bibek is a practice not only of rejecting food from non-amritdharis, but a bibeki Singh will reject food from amritdhari. The food is supposed to be prepared in a special way by chanting Naam while you prepare it. So bibek is secondly about putting the Naam, the power of spiritual liberation into the food itself so it has power to wash the mind and body.
Yes, the whole world is impure. But the Guru's Sikh is set apart from the world and given the Naam, the door to liberation because he belongs to Guru. Naam is what will purify the mind and body.
Kabeer says they alone are pure who obtain pure understanding. So who are those pure people and what is that pure understanding? Later the pauri goes on to say,
So the word of the Guru's shabad is itself the purifier that makes pure people. The Naam makes mind and body pure.
So in context of Gurbani, the Naam actually is what washes the mind and body of spiritual filth and all uncleaness. So the practice of bibek takes only that food which has been made into langar by an amritdhari who is also a Naam abiyaasi who is japping Naam while preparing the food. This practice is a way of intensifying the purification properties of the Naam by ingesting vibration of the Lord's praise with every breath and with every morsel of food.
Gurbani is saying chanting with one's mouth gives a spice, a flavor from the Sound current of the Naad, and eating other foods give false pleasure which ruins the body and is an opening for wickedness and corruption to enter the mind.
Gurbani is very clear that Naam is what cleanses the pollutions.
Here Gurbani is talking about saas giras simran, where every breath (in breath and out breath with Waheguru mantar and in every morsal of food (langar of food prepared with Naam abiyaasi japping Naam to put Shabad current into the food).
Here Gurbani is talking about all the doors being wide open. These are the spiritual centers of the body and includes the tenth door/dasam duar all being opened wide by the Satguru, and with practice of saas giras simran japping Gurmantra with every breath, and every bite of food. So Gurbani is clear that spiritual practice of bibek belongs to Gursikhi as a means of purification and liberation.
Guru gives you Naam to eat, to breathe, to open all your bodies doors of perception. The door of Naam/praise is that which you should go through. Gurbani cannot contradict itself. If you research there are also supporting Rehitnamay which support bibek.
