Of my understanding, the Book of Lorgar isn't so much ignored by the other legions as it is jealously guarded by the Word Bearers. They preach to the choir, because it works, but they do little to proselytize to the masses of Black Legion, Night Lords, World Eaters or Death Guard... for examples. Though, I'm sure that's also because the World Eaters have their own idea of what it is to pray to Khorne ("Hack and chop, gut and drink and we take a few heads as well, that's how we say our prayer to Khorne before running back into Hell!" ~to the tune of the Merry Ole Land of Oz), as do the rest of the traitor legions.
I find it interesting the title of this says "All roads lead to Lorgar"... I had assumed there would be some detailed explanation of how the Imperium will turn to worship Chaos Undecided like Lorgar. Instead, I find an argument about how despite being a polar opposite to Lorgar, Guilliman supposedly ended up doing the same thing Lorgar did. To this, I must protest.
Lorgar worshipped the Emperor as a God. He encouraged his entire legion to worship the Emperor as an immortal and beneficent god of all Mankind, and that the space marines were his vassals and stewards of the flock.
Roboute Guilliman has done no such thing. In fact, the Codex Astartes, in its many teachings speaks of the Emperor as a father figure to the Primarchs, who in turn are the fathers of their legions. Many of the chapters who have adapted the Codex Astartes have taken a secular view of the Emperor, with more of an ancestral veneration in their practices than any form of religious worship.
Roboute himself never believed the Emperor was a god (and I think the other loyalist primarchs held this view too), so, in essence, he never did follow the road to Lorgar.