This article's core argument seems to be that Pixelfed is violating the ActivityPub protocol by not displaying posts that do not contain images. That's just not true at all. I'm interested to know where the protocol ever has such a requirement.

The principle behind a communication protocol is to create trust that messages are transmitted.

And they have been transmitted. They've been filtered out after transmission, but the protocol did its job.

If a message is not delivered, the sender should be notified.

Perhaps. But that's not in the spec. There's no obligation to notify iirc that a post got filtered out on the target instance.

Even if Pixelfed sent Reject(Note) back for every post without an image, would Mastodon even display that to the user anywhere? Would most users want to see that for every post not containing an image multiplied by every Pixelfed instance it got federated to? I'd personally interpret that as spam.