I don't think it's a "secret" at all. Given the context of the actual statement...
“Your mind is closed to us,” said Lalla, “like a fortress preparing to resist an invader.”
“I don’t understand,” said Uriel.
“You Astartes, your minds are as rigid and unbending as adamantium,” said Kulla, and Uriel knew that her mouth was not moving. Her voice was arriving directly in his thoughts without recourse to speech. “You are trained, conditioned and enhanced in so many ways, but your minds are like locked doors to a place of miracles and wonder. All the potential you are trained to access: memory, language, combat analysis, and yet your masters train you to close off the one part of your mind that might actually allow you to soar. You do not feel as others do, but we can open that door for you if you let us.”
“Stop it, Kulla,” said Lalla. “You know that’s not allowed. Leave him to his blindness.”
... I'm pretty sure they are referring to the effects of the psycho-indoctrination Astartes are under. The same process that gives them some resistance to mental influence (as implied by Lalla, above) also deprives them of much of their free will, imagination, etc. That is, an Astartes is defined as much by his mental fortitude as he is by his physical prowess. If he were able to think for himself, to make his own choices, to have an open mind (as Kulla implies), he would no longer be Astartes.
This, by the way, ties in with the consistent themes the fluff has introduced on the Astartes. On the other hand, Graham McNeill would be rather remiss in introducing a dark secret regarding the Astartes which is neither found anywhere else in the fluff nor even mentioned again in his own novel...