It was carried forward from days before checksums were a thing for the main data files. Enabling them on existing clusters was not added until PG 14 (in core - there was a utility prior that that could do it, but not much before). So before this there was no real way to enable them on existing clusters short of using logical replication (which as a ton of caveats and issues).

Even today, enabling as in the blog is not officially supported as far as I know (but it does work - I have done it in the past across tens of thousands of DB clusters) and is far from simple.