I think they're referring to single-table data design as described in https://www.youtube.com/watch?v=HaEPXoXVf2k It's quite clever and scales well, but completely unnecessary for the vast majority of CRUD apps.
IMO it’s also difficult when you’re in the initial phases of a new company/product, because the system is evolving fast and that doesn’t play nice with single-table-design needing upfront design to work well.
I have had much better luck sticking to a plain old relational DB stack for those phases, because it’s fast to iterate, and then consider moving to single-table NOSQL when the system/DDD has “gelled” and traffic is starting to hockey-stick
