The quote is not true. I've personally done dozens and dozens of integrations, which can/are all be applied to any district. It's actually fairly simple with a functional programming approach, set theory, working with the k-12 companies directly, and if you think of them as data flows. I've taken over for some ex-fb/ex-google, large k-12 companies, vc funded ones... and every single one does it in a really coupled/tangled-mess manner where the statement is actually true (I take over their codebases). If done right, the integrations are a few hundred lines of code (even doing schools, courses, students, teachers, enrollments, grades, standards, etc). The only thing that changes between districts is typically a couple of business rules which are done as filters on the data flow. The unit/integration tests take more lines than the integration.

Example integration flow of SIS to LMS: SIS client (pull) <- business rule filters <- integration itself -> business rule filters -> LMS client (push)

Example integration flow of LMS to SIS: LMS client (pull) <- business rule filters <- integration itself -> business rule filters -> SIS client (push)

Examples of system clients: https://github.com/rockymadden/brainhoney.js https://github.com/rockymadden/masteryconnect.js

Example of K-12 specific spec that is often so mangled into most codebases it causes bugs: https://github.com/rockymadden/lti.js

Some I've done (almost a cartesian product): Infinite Campus, BrainHoney, MasteryConnect, Moodle, IFAS, Blackboard, Safari Montage, Edgenuity, netTrekker, PowerSchool, Canvas, dozens more...