@Pick-a-boo_doo I saw the same thing. After using the basement tool, if I expand the basement to the ground floor boundary and beyond, the terrain raised. I wish I had tested that in the the video. :-(
I also just tested building a room below first and the terrain never raised. It didn't matter where I put the basement room or where I pulled the walls. The terrain only raises when I use the basement tool.
I don't know if it's a bug or not. It's possible the devs actually did intend for the tool to cause the terrain to be raised as you suggested. It very well could be that the basement is just a bit closer to the foundation since since you're drawing the room directly below the ground level. Whatever the case, I don't think this thread will be moved to the bug forum. For one thing, bug reports need to have the template filled out which helps the dev team to know what packs may be involved, if any mods were used, what game version was used, etc. Also, this forum is for general game issues, not just for Mods / CC issues. So if you or anyone else believes this is a bug, I'm fairly certain there would need to be a separate bug report.