I’m running this tumblelog on a WordPress Multisite instance, so plug-ins work a little differently.
TIL that you don’t have to Network activate plugins. You can just install them in the network plug-ins manager and leave them deactivated, then activate them individually in each site. Do it that way and the plugin will behave as it would on a single WP site.
Therefore, ActivityPub can be available for all blogs that are part of Cordelya.net!