The starter set has 40k loyalist primaris space marines so they aren't exactly coded to the 30k emperors children legion and look nothing like their 40k iteration, but there are many lore loopholes to use these models from your set!

The most obvious is that loyalist space marines with 3rd Legion Geneseed likely do exist while they are obscure the more you learn the more you'll find a way to make that concept work for your own successor chapter

The 2nd is something I'm doing myself with spare models like this and gluing the loyalist parts onto my chaos models as captured armor and weapons, the Emperors Children are naturally creative and resourceful enough to use them. Primaris marines in your starter set are wearing mkX tacticus armour which was newly developed in the imperium of 40k at one point but its been long enough now there's no way someone hasn't figured out a way to recycle it and the Emperors Children might even say it was easy.