For what it's worth, I find the availability & typing indicators in Slack to be unreliable. When someone "is typing" for a long time I assume it's lying. In this circumstance, it's likely they needed more information or otherwise weren't able to answer your question right away, and they have deleted their message & are not actively typing.
This is a great "feature" of slack, because when somebody accuses me of ignoring then citing "is typing...", I can tell them it's Slack's fault (and half the time it is!)
I don't know about Slack, but ones I've used in the past would continually show that indicator if you accidentally hit a key and leave something in the input box, but have tabbed away.
