You can put them in the interface, and then you can use the interface as a type constraint for generics:

https://go.dev/play/p/U25AzZndz-7

If you wanted to argue that you can't use it everywhere you could use an interface before, that'd be all well and good—but your assertion

> you can't actually put concrete types in interface{} definitions

is just incorrect, in a way that lead you to misunderstand the article.