I don’t know enough to say if it’s more insular or not, I don’t know how common it is to have the default sort as All, but we’re definitely worldly enough for other instances to have some users pushing stereotypes on us when we comment.
You do have some point about lemmy.ml having enough instances that you can get by with Local as default, but I assume most people would be subscribing to or exploring other instances too? I really don’t know.
The fact that Lemmy doesn’t calculate and show a user’s total post score is proof itself that it doesn’t reflect Lemmy’s political leanings.