If the Granth is truly from God (rather than about God) why does it contain the works of men e.g. Kabir and Farid, when God is far more eloquent than men and can provide more guidance and instruction with His words?Okay, so, I would begin with saying the 10 Gurus are believed to be made from the same substance as God, I've heard the analogy "the Guru's were the drop from the Ocean of God" so therefore Guru Arjun Dev Ji (who himself is made from the same substance as God) adding the Shabads of Hindu and Muslim Saints, is not man conveying the message of God as you put it, but rather it is God conveying the message of God through men
to other men. I see it this way, say you are a devoted muslim and a a 65 year old Jewish lady who only spoke Yiddish told you that she had the "Word Of God" and the "Complete truth" in her hands (in Yiddish) you would accept her word for it? Most likely not, 1) You would have a problem understanding her 2) You would not accept it since you are already faithful to Islam for example and you have no reason to accept her word for it.
Okay so now, say a 20 year old (around your age), well known,well loved and respected, very intelligent English speaking Christian man came to you and told you he knew the truth about God and wanted to translate it through poetry or a song to you, would you be willing to listen? it's far more likely.
This is how I see those saints, not equal to Gods word own words which he spoke through the Guru's but rather, Gods teachings translated to the laity by men to men.
All of the men share very similar beliefs in humility, simplicity and One true God regardless of their faith and background, so in Sikhism's eyes they are Sikh since they follow those Sikh principles and this would reflect "the universality of thought which underlies the sikh belief in One God" and of course, almost all of them were well known and praised for their work and where respected so that would be more reason to include their words.
It was Guru Arjun Dev ji (a substance of God) who ultimately included their work into the Guru Granth sahib ji which means God saw his truth represented in their words and saw reason for it to be incorporated amongst his own words, it doesn't mean God can't do the same, God himself had already spoken through the Gurus (and had yet to speak which is why blank spaces were left by Guru Arjun Dev Ji).
In my spoiler I've highlighted the Bhagatas (Saints of various faiths) and their similarities with each other, which really conveys my point of them being examples of men who saw the "Truth" that the Guru's spoke and practiced sikh beliefs themselves, and it's clear how their commonality, skill, love and respect with and from ordinary people would help ordinary people experience God and the Complete Truth that Sikhism holds.