The channel that you choose when setting up the “incoming webhook” on slack is not necessarily the one that discourse will send posts to. (I’m not entirely sure why Slack asks you to specify a channel, then ignores it)
To change where discourse sends the messages, you need to change the setup in the “Plugins”/“Slack” section of the discourse admin panel. Delete the existing filter (which will be sending to #general), and create a new one for the channel of your choice.