Same as in #47406 (review):

AsTaggedItem is supposed to work together with TaggedIteratorArgument. That's what the word "Tagged" means in the name of the attribute. When not requesting for "tagged" items, the returned array should be a regular list.

#[AutoconfigureTag('some_tag', ['priority' => 10])] is what you might want to add to set the priority for this tag.

If we were to implement something like you expected, I'd suggest doing it via a tag, eg
['default_attributes', ['priority' => 10]], and we could of course add a dedicated attribute to make it easier to define this tag.

Or maybe ['default_attributes', ['some_tag' => ['priority' => 10]]] for more specificity?

Or something else? I'm just thinking loud here :)

#[AutoconfigureTag('some_tag', ['priority' => 10])]

This works just fine for me.

I'm still not quite understanding when/why I would use AsTaggedItem over this though. Most of these attributes aren't documented so that could probably be the place to explain.