My intent was not to identify a viable channel for spamming the fediverse, but more to point out that permitting messages from random new nodes happens in at least one standard interaction on an activitypub server - an ‘existence proof’ of a sort for a message channel that doesn’t get solved by follow-based whitelisting.
Economics effects still apply to spam. Low useful signal still is an economic factor on if it is worth to do follow request spam "at scale" or not.
Also, federation itself may be allowlist-based. Some instances already run that way. Likely many more would if inter-instance spam follow requests rose above a certain threshold.
