Short answer: No.
As the documentation specifies, the list
action "Returns all tasks in the specified task list".
The "restful" style is that all resources belong to a collection. As such, using list
for Tasks requires acting it on a TaskList. As you noted, it's very simple to list all TaskLists, then list all Tasks from each TaskList and collect them in a list. Note that one might even argue that in many circumstances, a Task is meaningless when separated from the context of the list it was created in.
You can see the full Api Reference for Tasks API here.