Unfortunately, they leave as something "for further research" the potential use of surrogates.
I'm not going to do any modelling here, but I really would have expected that celebrities would disproportionately use surrogates. Surrogates aren't cheap; celebrities have lots of money. Surrogates might make unobservable decisions about nutrition, substance consumption, or activities that underweight the fetus's interests; a celebrity can afford to have the surrogate live in their house for the period from IVF through to birth. And, while celebrities might have greater incentive to demonstrate altruism than do us normal folks, they're also more narcissistic - which should push towards stronger preference for their own genes.
I really don't follow celebrity gossip. Maybe there's lots of celebrity use of surrogacy options that I've just not caught. But the only case that comes to mind is Patri Friedman, if he counts as a celebrity. On celebrity adoption, it's so common that Sarah Silverman was able to lampoon it in her TED talk.
The only answer that makes sense to me is that surrogacy (alas!) remains subject to disapprobation while adoption draws kudos. And, Bethman and Kvasnicka hint at that answer:
For a celebrity, the adoption of a child generates headlines and adds to popularity, which benefits the career and ultimately income.
A celebrity expecting "rent-a-womb shocker!" headlines in response to a rational decision to avoid personally incurring the physical costs of pregnancy might well be tempted to choose adoption over surrogacy.