It's important to note our understanding is far from complete. There may be more genes associated with autism than we currently know. So 30% of cases may have a known genetic factor but that doesn't mean 70% don't have a genetic component.
This is also explained as a genetic factor as chromosomal abnormalities and replication errors are more frequent although cumulative environmental exposures may also play a role.
That's what our genetic councilor said as well. It's important to note autism is a spectrum and quite varied, some of it genetic and some of it related to other factors.
I've read some studies which suggest there's a variety of genes which are linked to autism as well as link to both autism and ADHD. I believe those genes are linked to how different brain circuits interact.
It makes a lot of sense given with I've seen talked a lot about in autism and adhd groups, with some symptoms overlapping.