Technically, you can demand an immediate response via email (and in some industries, this is quite common). It's the expectations around the channel that matter.

Agreed, but I would say that chat channels (and especially 1:1 chat pings) typically have expectations closer to immediate mode compared to other mediums. Because they typically cause a notification and break in concentration, I label them as "synchronous." Any communication medium that does not have notifications and only works via me polling it is asynchronous.

a lot of things matter. Just the fact that there is no pre-loaded text area to respond changes the dynamics of the conversation.