That's correct ...
For Help on this, when you're in the compose message screen, if you click on the
Help button at the top corner of the screen, the pop up window that describes all the features says, under 'Preview or Preview / Delay Post':
The preview button allows you to see how your message will look before you post it. This is ideal for when you have included some formatting to the message to ensure that the post will look the way you wish, and that you have entered in the codes correctly. Additionally, GMs can delay posts with this option, selecting the appropriate time delay in the preview screen. Note: The delay post functionality is performed by your javascript-enabled browser. Not only must you have javascript enabled, but you must leave the browser on this window while it counts down the delay and eventually posts on your behalf.