A task can be in one of the following states

like the one in the picture.

Figure 2: Refreshing an Expired Access Token

Here’s what this flow looks like:

As you note, Activity diagrams inherently can include concurrency and timing. If you look at this example cribbed from Wikipedia, shown below, you can observe the section with two heavy horizontal bars, and two parallel activities of "present idea" and "record idea". That is read as "start these activities in parallel, and continue only when both are complete." Flowcharts can't express this within the notation. Practically, using activity diagrams lets you think clearly about concurrent processes. I think you'll find that anyone who can read a flowchart will quickly adapt.

specific types of diagrams are also called a type of flow diagrams

The uses of flow diagrams are vast and honestly endless.

+----------------------- | RESOURCE EXISTS ? (if private it is often checked AFTER auth check) +----------------------- | | NO | v YES v +----------------------- 404 | IS LOGGED-IN ? (authenticated, aka user session) or +----------------------- 401 | | 403 NO | | YES 3xx v v 401 +----------------------- (404 no reveal) | CAN ACCESS RESOURCE ? (permission, authorized, ...) or +----------------------- redirect | | to login NO | | YES | | v v 403 OK 200, redirect, ... (or 404: no reveal) (or 404: resource does not exist if private) (or 3xx: redirection)

Here comes a sample screenshot.