Just to let you know the mahapurakh that told me this was pooran and is very well known to the panth. Brahmgianis do not "brainwash" you rather they give you true gian of gurbani NOT JUST THE MINUTE GIAN YOU GET OF THE ENGLISH TRANSLATIONS OF THE INTERNET.
Singh I humbly request that you dig deeper into gurbani rather than just plugging english translations of the internet your not interpretating them right.
The reason is that the term satguru doesn't just mean the sikh gurus only it means any avtar, pooran brahmgiani etc. The term satguru has been discussed by mahapuraks many times and even brahmgiani sant jagjit singh discusses the meaning. Please please listen to the veechar it can be found online.
Now replace the definition of satguru with what the brahmgianis say now it makes more sense doesn't it ji?
Sant Jagjit singh ji, sant isher singh ji, sant dalel singh ji, sant harnam singh ji, sant karam singh ji etc all mention satguru was in the existance in nirgun saroop from the beginning. They came as sargun saroop of ram chandar, krishan avatar, Guru avtar etc to spread naam. All avtars were pooran brahmgianis in their past life before they took form as avtars. For e.g. Guru gobind singh ji's past incarnation was dusht daman and his "satguru" was a rishi who gave him "nam" or charan paul amrit. Then once he became 1 he came as guru avtar in kal Jug.
Also if you have a chance to study any of the brahmgiani's writings above you will come to understand this. I used to think sikhi is the ONLY WAY. But after veechars and getting personal bachans from pooran brahmgianis daas has noticed how universal all brahmgianis are just like how universal waheguru is for everyone not JUST SIKHS. Also this is what sant jagjit singh ji said when a premi asked them that are sikhs only accepted to meet waheguru(Sachkand)? He said no, anyone from any dharam can. Making all the avtars sachkandis themselve otherwise the followers of another religion cannot meet god if their avatars won't sachkandis. Santji further explained that if waheguru favoured the sikhs more then he would of given the sikhs further privileges such as wealth while ppl of other religions would of been made poor, given them water while other religions would of been left thirsty and hungry. Clearly this is not the case and my point being all avtars and guru are the same comparing them is futile and all religions are equally pleasing to akal purakh.
Singh Ji you need to listen to veechar by brahmgianis as their knowledge is true and the english translation don't compare to their knowledge that are found on the internet as it was not written by gian vadis. 1 premi once asked how much more knowledge does a brahmgiani have compared to a person that is agiani? The answer given was a metaphor as the knowledge comparison can't be described. The metaphor given was that the agiani's knowledge is equivelent to a drop of water while a brahmgiani's knowledge is equivelent to the vast water of the oceans!
Also 1 more thing all the avtars and guru avtars were not born nor did they die. Veechar is done on this as well so please spend your time learning what this means. Look out for how the avtars bodies differ from ours and how they came into existance etc. You will get your answer.
So please please listen to veechars and educate yourself.