I can't say with any real certainty I'm afraid so the rest of this post may be complete tosh. But I'm not sure he's affiliated say in the same way Simon Stone is. Nor does he likely have a high level source like Castles (Mourinho). But he's head over heels in love with this current United team for some reason (used to be City when Mancini was in charge I remember) and he's not hid that bias in any shape or form, so for him to willingly produce such a negative and potentially disruptive article and go completely against type suggests he's heard something more concrete.
I'd wager someone out there has reached out to him specifically with this information for the reasoning I outlined above. That is, if United lover Custis says something this bad it must be true.
It won't be someone directly from United as he's not Stone (in fact they've been feeding him the 'all is well' line and he's been churning it out like no tomorrow). We also know it's not United because we know they have no interest in letting Pogba go. It won't be Mourinho as he's not Castles. If it were the article would have been far more critical of solely Pogba like a few others have been. He's not such a reputable hack that someone with a bit more info in let's say France would let him know first. Which leads me to believe he's got his information from the Pogba camp in some way or form, likely at Mino Raiola's bidding, if not directly from the man himself.
That might seem a counter-intuitive conclusion btw, considering the article puts plenty of the blame at Pogba's feet and Raiola's too as well as Mourinho's. But at this stage with the close of the transfer window drawing ever closer Raiola is just going to try and be as disruptive as possible is he really is angling for a transfer and this was a nice way to do it by making out everyone's contributing to the 'carnage'. And as I say, it may not have come directly from the Pogba camp; they may have just ensured the information found it's way out in one way or another.