It's hardly the most glaring issue with the timeline Laurie Goulding presented.
But it kind of is a really big one now IMO. Part of the reason Monarchia and the Emperor's censure made such a big impact on Lorgar is because he had been crusading in that manner for over a century. And Monarchia happened roughly 40 years before the Heresy. Meaning Lorgar himself had to have been around for over 90 years before the Lion was found. Which we now know is impossible seeing as the Lion was found before Lorgar.
On to Horus though, it seems he had rivals in Dorn, Guilliman and the Lion, in terms of skill as a military commander. But like others have noted, compared to those 3 he was far more charismatic and enjoyed far better relationships with the other primarchs than they did. Guilliman himself notes in Know No Fear
, I think, that there are very few primarchs he truly considered friends. We know how the Lion is in terms of keeping secrets and trusting others, so I am inclined to think many primarchs were mistrustful of him, even if they did respect him (like Guilliman did). Dorn I think would be very similar to Guilliman in this regard. Kind of aloof, and slow to make friends with the other primarchs. So from that point of view Horus was a better overall choice to take on military command of the GC. None of the others who could be considered his peers in terms of military or logistics could compete with him in terms of his ability to handle his brothers. So as a military leader (including handling potentially difficult subordinates), he really was the most well-rounded amongst the primarchs.
In terms of singular combat, I also would rank Horus high up there. If we consider that Corax, who considers himself among the best fighters of the primarchs (confident enough in his own abilities that he thought he could kill both Lorgar and Curze at the same time), thought only Horus and Sanguinius stood a shot at beating Angron (where he was positive he would fail), we can put Horus up there. This is also reinforced in Betrayer
where Lorgar says, in a kind of reverse scenario, that only Angron or Horus could stand against a bloodlusted Sanguinius. So in terms of combat (in general, barring having favouring circumstances) I'd probably put Horus at the top tier there, along with Angron and Sanguinius.
On a random note, Ferrus Manus seems to be more competent than most in terms of military leadership too, if The Unremembered Empire
is anything to go by. Guilliman listed him along with the likes of Dorn and himself and the Lion as potential rivals for the Warmaster spot, when he explained to the Lion that Horus cared the most about besting him for the spot above any others. And later when pondering what to do with the shattered legions, he immediately thinks to put the Iron Hands remnants as the base command structure, and then worries if the other legions would take orders from them. And Guilliman would only be considering that if he viewed the Iron Hands as the best choice to lead the remnants. But I am wandering off topic now... sorry...