That is correct. You will need Mojave 10.14.6 in order to download current iMovie.
Here's a link that will explain how to upgrade to Mojave, and will list the Mac models that are capable of downloading it, as well as the memory and storage specs needed.
https://support.apple.com/en-us/HT210190
If you have, or ever have had iMovie 10 on your computer but perhaps deleted it, you can download the most recent iMovie update compatible with your operating system. To do that drag your iMovie app from the Applications folder to the Trash, but do not empty the trash. (You can eliminate this step if you have deleted IMovie in the past.) Then sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see a folder with your purchases in it, including iMovie. Update iMovie from there. If the update is not available or is unsuccessful you can drag your present iMovie app out of the trash and put it back into the Applications folder.
-- Rich