There’s certainly confusion about this, as Slack can be used for both.

If someone sends you a message, and they’re happy to wait for a reply, then that’s asynchronous.

If someone posts an @here in a channel asking for comments before making a final decision on the spot, then that's synchronous.

Yes, and this is exactly the problem with Slack: it tries to shoehorn both modes into a single mechanism.

Asynchronous communications must be structured fundamentally differently from synchronous ones in order to be effective. Trying to merge them into a single mechanism is a horrible mistake.

Async communication is what email is for! People who use Slack for sync communication are hopelessly confused (obvious tell: async doesn't notify you). This obviously only applies to DMs, @heres, and tags: public channels are just async modes that can smoothly transition into sync

I always treat slack as asynchronous.

I find phonecalls less disruptive, they happen once, and the issue is addressed. If it is dragging too long, we move it to a meeting.

A chat session can continue indeterminate amount if time, keep flaring up and distracting you as different people chime in from half-afk half-paying attention standpoint.