Apple's app store is a walled garden. The web isn't a walled garden. TFA wants to be able to operate outside the walled garden.
EDIT: Also, it's probably cheaper to develop one PWA than a PWA + N native apps, even if N=2. Probably lots cheaper. Now, perhaps there's a way to build a native app that is just a wrapper around WebKit/Safari and a PWA, but you'd still be subject to Apple's walled garden. For example, think of Gab or some such website whose apps have been banned by the various app stores...
EDIT: Also, it's probably cheaper to develop one PWA than a PWA + N native apps, even if N=2. Probably lots cheaper. Now, perhaps there's a way to build a native app that is just a wrapper around WebKit/Safari and a PWA, but you'd still be subject to Apple's walled garden. For example, think of Gab or some such website whose apps have been banned by the various app stores...