That's extremely not true at all. Even in the first trilogy, Horus internally monologues about several of his brothers that think they should have been Warmaster over him. The Lion even blatantly says the only reason Horus was chosen was because he was the first one found by the Emperor. Guilliman wasn't even the greatest tactician in the Horus Heresy he was a statesman. The Lion, and to a certain extent Dorn and Perturabo hold that role. Yes Horus brought the most worlds to compliancy (then The Lion then Russ then Guilliman) but you have to remember he was also the first Primarch reunited with his legion so had a longer Crusade than anyone else. As for combat, Horus himself internally monologues that in single combat he thinks he would be defeated by several of his brother. It was only the ruinous powers that allowed him to would the Emperor which was very parallel to Luther being able to would The Lion.