This is confusing, and non-intuitive
Batch edit is a feature specific to Maniphest, and therefore it makes sense that is tied to Maniphest's search only.
Also, Maniphest search only provides tasks as results, and therefore batch edits are always possible because all tasks share the same attributes and actions.
General search can throw tasks, mockups, pastes... and in the future Diffs and perhaps more. Even if batch edits would be avalable for these other objects, they have different attributes, which limits the chances of functional batch edits. Also, it is unlikely that someone needs to apply the same action to different types of objects.
The current behavior is consistent, and I'm taking the liberty to decline the task because I'm sure this is what will happen upstream.
That doesn't make sense to me, any modern system could look at the results and only show bulk actions that are relevant to the query results. Both tasks and mockups have projects, comments, CCs, a valid subset of bulk actions could be shown.
Applying a project, comment, or cc to a mixed set of results seems like a completely valid use case.