Ant's playbook looks much like Amazon's: they are their own first-and-best customer in each market. For every product marketplace they build, they bootstrap it by providing their own offering, first. Be it an SMB neo-bank (though MYBank was also initially part of Alibaba before being subsumed into Ant) or an asset management fund. You can even see this in the growth of Ant itself: Ant's first-and-best customer is Alibaba, whose needs and data drove subsequent innovations at Ant.
This makes a lot of sense. Firstly, it's hard to convince legacy financial institutions to become tech-first. Secondly, the platform gets well-tested before being opened to third parties. Thirdly, once the doors are opened, Ant transforms from financial entity to platform, freeing the company from pesky financial regulations (the reason behind the initial rebrand from Ant Financials to Ant Group). Lastly, it gives Ant a first-mover advantage with customers. It's not surprising that Tianhong is still the largest asset manager on the platform.