@blog From a quick read of the code:
Around https://gitlab.com/edent/activity-bot/-/blob/9fed3dff8b3f02c836a1e48ae02b40c3d74a057d/index.php#L815 Note construction seems to be missing the cc field, which is present in the Create activity, they need to match.
Around https://gitlab.com/edent/activity-bot/-/blob/9fed3dff8b3f02c836a1e48ae02b40c3d74a057d/index.php#L1039 means the mention part of the bot is Mastodon-specific as other servers don't use https://{$domain}/@{$user} and also will likely not work properly as it doesn't seems to add their ActivityPub IDs to to or cc fields.
Might be a better idea to have the bot client mention via raw ActivityPub IDs instead of @username@domain since that requires keeping a user database.