The only other thing I could think of to prevent this would be a DMCA 1201 claim - i.e. that the view source link is a DRM system, and that a proxy that removes those links is illegal. Except all v3 licenses explicitly foreclose the ability to make DMCA 1201 claims relating to features of the software.
My guess is, Mastodon used AGPLv3 primarily because the Affero clause scares off big tech companies, notably Google. I don't have access to any IRC logs or issue tracker comments to prove this is the case, though.