If I like something it goes to the distributor server only, and that server takes care of relaying the activity.

That's only half of the equation, though. You send an activity to the distributor, okay, that's one recipient. But then the distributor ostensibly distributes it to more than 1 recipient, right?

Traditionally, yes. If I like something it goes to the distributor server only, and that server takes care of relaying the activity.

Implementors that straddle threadiverse and microblog send it to all followers and the distributor server.

it’s not allowed to be an array, according to the spec

Where are you getting this information? Common fediverse softwares might not handle it correctly, but the spec makes no requirements like what you say. In general, anything not marked "functional" is theoretically always an array, even if it is serialized as a single value or object/reference when the array contains exactly 1 item.

Mastodon-style federation where a note can go to multiple inboxes but that very rarely happens in 1b12 land.

What is "Mastodon-style" about this, and what problems does it pose for 1b12? Are you saying that in FEP-1b12 implementations, activities are only delivered to a single recipient? That doesn't sound right.

object is a collection because it’s not allowed to be an array, according to the spec. Initially it was an array (and PieFed still accepts that) but discussion with other 1b12-using devs convinced me to switch to a collection.

fep-4248 is an older name, I’ll fix that!