As I have mentioned before, we have worked with both typescript and flow. Flow has some theoretical advantages, but the mindshare of typescript as well as the responsiveness of the community is just orders of magnitude better with typescript.
Flow sometimes seems like some facebook internal tool that they just put out there as open source, but they don't really care if it is used.
I strongly feel that it would help the adoption of IPFS if there were typescript type definitons for the API. What is used internally in the code base is completely separate from this.
Here is an issue I just opened: https://github.com/ipfs/js-ipfs-api/issues/864