There is not a huge amount of in-depth information in the press release, but Jason Cooper goes into detail in a comment on this ManageWP post. Jason notes that “at the end of the day, we simply saw the opportunity to work with a fun new technology”. It would be nice to see this someday be available for sites beyond those hosted on WPEngine.
WPEngine has announced “One-Tap Payments” for WordPress ecommerce sites. Their WooCommerce extension leverages the Payment Request API — which is already available in the mobile versions of Chrome for Android.