I suspect this is because the admin track is still being tested and is not considered quite ready for prime time. The big priority is the new user track.
I just tested this by setting it to 20000 seconds, and the delay seems to be working on my site. Discobot's message is not showing up. It's true that it's a bit weird for the screen to turn black immediately if there is no notification there to look at.
However, I have another welcome message arriving via SSO so would prefer the screen darkening feature to not go away. Could suppressing it be added as an admin option for those who have the delay set and don't need it? Or wait to darken the screen when the discobot message is sent?
Also, the max delay appears to be 20,000 seconds or about 5 hours. I was hoping to try delaying it by a day or a few days to use it to remind people to come back to discourse. Could this be done?
And finally, if there is a delay, then it will be sent out by email since the user is probably not online. So some noobs will be confused and will try to click the
... in their email client. I had this happen a few times already. I don't know if there is a solution for this.
(For my site, I'd like to see the discobot message go out immediately and be highlighted with the screen darkening, and then send out my SSO-sourced welcome message a few days later. This is something I'll have to try to figure out on my own since it involves updating some custom code that @AdamCapriola wrote for me back in the day)