Your argument is equivalent to saying that ISPs can "outsource" email to third-party services like Gmail and Outlook. That is an awfully stretched interpretation of what it means to "provide" or to "outsource" a service. ISPs do not need to coordinate with third party DNS servers to have their customers use those servers, any more than they must coordinate with third party email providers.

There are plenty more falsehoods in the order. The order states that DNS is analogous to a gateway server that translates addresses and not analogous to a directory service. The order claims that transparent caching is a critical aspect of ISP service that users have come to depend on, and dismisses comments pointing out that numerous modern web standards break transparent caching. The order states that the service an ISP provides is a "multi-user computer server" through which consumers access the Internet and ignores the fact that most consumers receive public IP addresses and are technically connected directly to the Internet (and that, technically, it is possible to host edge services using a broadband connection, even if doing so is rare). You can go read the order if you want more examples.

"It's not as if they are saying something inexcusably erroneous, like for example that JSON is the protocol used to route packets."

They also suggested the DNS is like a proxy server (in their words, a "gateway") rather than a directory service. Does that count?

"Any other technical inaccuracies?"

That consumers continue to rely on transparent caching and that caching is a core ISP service. The FCC dismissed comments pointing out that TLS breaks transparent caching on the basis that there are websites that do not use TLS.

The order also claims that because people are able to access websites via an ISP network, the ISP provides people with the capability of whatever those websites do (e.g. under the order's reasoning, Verizon is providing me with the capability to have this conversation with you). You can argue that is an opinion and not a fact, but the order does not apply it consistently; for example, it does not assert that a phone company is providing an information service by virtue of its customers' ability to use a dialup ISP.

The order claims that by connecting to your ISP's network, you are receiving, "...computer access by multiple users to a computer server...that provides access to the Internet." Maybe that is just how the FCC interprets routers, but again it is not being consistently applied e.g. to the phone system.

If you want more, go read the order; the technical analysis is not very long.