40k space marines are better than 30k space marines. They spend decades as a scout learning how to fight, becoming familiar with all their wargear, then they usually go for vehicle training after getting their full panoply, then they spend yet more decades learning specialized warfare before becoming a tactical marine. Their armory became restricted so they had to learn how to fight without all the gizmos the legions had access to and without the numbers to throw at the enemy while still having to accomplish the same tasks. The codex astartes just made sure that on top of the hypnotic conditioning that the space marines got they were painstakingly trained and molded into proper warriors. By the end of the heresy, even discounting how much of it was flash trained inductii chaff, how much of the legions had not fought the xenos and had just learned to fight fellow space marines? Sure fighting space marines is tough enough but it's not the same as fighting the ork hordes or the freakish horrors that were never really destroyed.
Guilliman ensured that space marines would be true to the Emperor's vision instead of just being one-trick muscleheads that were only better equipped mooks than the mortal armies.