There are lots of smart optimizations I could use to do that (clear roles, efficient meetings, managing task intensity), but there is this general problem of who is holding the risk in the business.
If you miss a client commitment or the sales people want to blame losing a sale on someone else, the first place they are going to go is the 30hr person "not pulling their weight."
In development, I think most pro developers would be massively more productive if they didn't have to waste away in an office 8h/day. But when you get out into the field where the business is about relationships, which are in turn about perceptions and commitments, in an enterprise environment, I guarantee a customer will say, "nice product, but if you can't demonstrate your ability to get me feature X by next quarter, and this competitor can, I'm going to scuttle this deal."
The salesperson whose commission is riding on this goes back to the product manager and says, "We're going to lose this high profile deal the CEO is watching because your engineers are soft and lazy? wtf?"
The PM goes to the CTO and says, "the survival of the company depends on deals like this, I need your people to burn the midnight oil."
CTO goes to engineering manager, "so this 30hr/week person, get them full time or get them out, we can't have this liability."
Eng manager says, "but they are my best developer! This is how we attract and retain the talent that makes this possible!"
CTO says, "get it done."
Eng manager writes email and starts the manage-out paper trail to 30hr person, "as per my last request to get these items done, we need to improve your performance..."
30hr super-developer receives weirdly formal email from manager, updates linkedin, starts pinging recruiters.
I'd wager 30hr weeks are a great way for startups to get talent they couldn't otherwise afford, but it will be temporary in their growth phase, as the above scenario is inevitable once they have momentum.