I don't know the paper you are talking about but the author of this one (David Spivak) does a ton of research on Polynomial Functors and he has a whole book on them: https://topos.site/poly-book.pdf

Poly is an incredibly rich category which can be used to describe a ton of stuff such as finite state machines, neural networks, game interactions, wiring diagrams and dynamical systems more generally. I wouldn't be surprised at all if Poly can describe database migrations.

> can be used to describe a ton of stuff such as

One thing that bothers me about applied category theory is that this (being able to describe stuff) always seems to be sufficient for people to claim it is applied. But if the people doing the applying don't find that framing particularly useful, then to me it does not count.

We've used Poly to prove results about functorial data migration.

Hey you might be interested in this work-in-progress type theory for Poly: https://github.com/toposInstitute/polytt

That might be a case of the author be really used to a certain crowd and writing for them. The work by Spivak on databases is well-known in ACT so the connection comes pretty easily. Also, in fairness, is pretty easy to Google your way around that claim.