I don't think Guilliman had any idea the primaris project would be as extensive as it was.
I mean, he handed the uncorrupted primarch DNA to Cawl WHILE HE WAS WRITING THE CODEX ASTARTES. Guilliman would never go 'hmmmm yes I have just written an incredibly thorough book of military reforms for this superhuman army. Now let's ask this mollusk of a man to completely rebuild this military force. I am very smart'
There's no way he intended Cawl to redesign the adeptus astartes from the ground up. When he told him to 'improve' the space marines he probably just meant, like, clean the gunk out of their gene-seeds, try to fix some of the genetic flaws, and maybe make a few improvements to their wargear.
But then Guilliman fucked off and died and Cawl was left unsupervised. For ten thousand years. And after cracking out pretty much everything Guilliman instructed him to do in the first millennium or so, Cawl got bored.
'You've done very well Cawl, your work has exceeded all expectations' can very easily be interpreted as diplomatic niceguy speak for 'what the FUCK man what the FUCK!!!!!'
And given how quickly Guilliman spread the primaris marines out across the entire indomitus crusade I think he was fucking terrified
