I’m not sure I understand what issue Linus et al. are trying to solve. If the full hash is used whenever a commit reference is saved somewhere, then why does it matter how core.abbrev is configured?

What are you referring to?

The article is about abbreviated forms, not full hashes. The Linus quote is specifically about abbreviation.

[Linus] He recommends kernel developers use 12 characters

For a large code base you can expect to further grow continuously for a long time, it makes sense to already use more than a minimum abbreviation so that you references remain unique, even if a decades time.

Configuring a wider and explicit abbreviation width that will remain constant is preferable because the displayed references are what you will copy and reference. It doesn't make sense to work with shorter abbreviations locally, but wider abbreviations when communicating with others. It'd be a hassle to translate and risky to miss doing.