Using Logic:Harkiranji states that the truth and God were always there, as one, then there is the other opinion that the truth superseded God. As humans, we are logical beings, and I do believe Guru Nanak pursued logic rather than superstition and miracles, so how can the truth that he attempted to teach us not be logical and simply understandable? The statement that God discovered the truth therefore needs more explaining.
God is all that ever existed or will exist - unity - formless (Nirgun)
If truth existed as something separate then there is not ONEness so truth was not discovered but just IS. Since God is all there is then God IS that truth.
All else happens within God - duality (Sargun formed expression of the ONE). Truth is within God. Truth IS God.
Mool Mantra is very clear on this.