Not that attrs or dataclasses has particularly significant attack surface, but when considering stdlib vs. 3rd-party you also have to consider the amount of maintenance and the release cadence. Attrs can release every few months if the rate of change demands it, whereas the stdlib has a fixed yearly release schedule that is tied to interpreter versions. Attrs has a small, focused development team whereas the stdlib is maintained by developers who are stretched very thin, and many packages within it are effectively abandoned. Upgrading dataclasses means upgrading everything in the stdlib at the same time, whereas attrs can be upgraded independently, by itself.

Supply chain attacks are a complex and nuanced topic so there are plenty of reasons to be thoughtful about adopting new dependencies, but it's definitely not as simple as "just use the stdlib for everything".