Assuming Apples takes their cut from gross revenue and not margin, which I believe is true.
Apple's cut: $30
Costs (including dev and mktg): $40 (60% net profit)
Profit: $30
Users Will Pay: $100
Dev Profit per $100: $60
Apples Cut/Profit: $30/$29.99
Dev Cut/Profit: $70/$42
> Dev Cut/Profit: $70/$42
Apple's cut is $30 but their profit certainly isn't $30 minus one cent.
Obviously there's the hard cost of transactions, which includes but certainly isn't limited to merchant fees. If it's an international app, there's the currency exchange involved in selling that app in local currency, under local merchant rules. Apple also absorbs various costs caused by avoiding and dealing with fraud. The transaction could be performed with gift card, which means maintaining that network, physical manufacturing costs, retailer profit margin... I seriously doubt that Apple even breaks even on the 70% when the payment uses a gift card.
There's the work Apple does in policing their store. While it does irritate developers, the end result is that the App Store is significantly less overrun with scams and junk than it would otherwise be—which means that customers are going to be more trusting of your app than would be the case if the store was a total free-for-all.
Then of course there's a subset of consumers who discovered your app because it's in Apple's app store. Providing you these eyeballs is valuable and worth something.
And of course there are a lot of soft costs—Apple did put a lot of work into all the resources for developers. How you personally believe these should be accounted for is obviously a matter of personal opinion, but that work does cost money and has benefited developers. There's an argument to be made that some amount of it could be assigned to Apple's app store revenues.
The physical gift card network is likely vastly more expensive. Not only are there frequent sales on these cards (I have no idea of why or how the economics of them work) but the cards need to be printed and the retailer will take a (small) cut.
Lastly, paid apps subsidise free apps. There is a ton of free apps, both from small indie devs and big businesses. They all pay $99/year. I'm pretty sure the bandwidth cost for all of Google's apps is a few orders of magnitude higher than that.
My, relatively unfounded, suspicion is that the App Store has a profit margin of around ~60%. Apple overall seems to strive for a gross profit margin of around 40% and it seems likely the App Store is doing a lot better. Of course this highly depends on what you consider part of the cost associated with the App Store.
In reality it's too complicated to figure with a simple formula. But, he was right if margin is a fixed cost (like, all initial development). If margin is purely an incremental cost (like network costs that increase with each sale), then he would be wrong.
For decades, developers were lucky to see 50% of the retail price. More likely they'd see 20-40% after the retailer and distributor took their cut. And then this 20-40% has to pay some very expensive licenses for software development tools (we forget that these tools used to be commercial software) as well as licence fees for any toolkits you might have used (such as Unreal Engine).