Many users have a poor understanding of what Flakes _actually_ are: They are a bit of UX glue on top of existing Nix features:
- Input/output schemas for `flake.nix`
- A lock file format
- CLI features to work with the two above
It's entirely feasible to build out a Flake user interface & evaluation support on top of Tvix without making it a first-class evaluator feature. See https://github.com/edolstra/flake-compat for prior art.
The key point of Tvix _not_ having support for Flakes is to not make special snowflake evaluator features that are tied in with it.