With images, you can do Metadata > Read Metadata From File, and LR will reread the metadata from the file or its XMP sidecar. But that command is greyed out for videos for some reason -- not all that surprising, considering that LR's support for video metadata is buggy and incomplete.
Have you assigned keywords, captions, or titles to the videos, either in Aperture or LR?
yeah, the 'read metadata from file' was my first go-to but no luck.
I haven't assigned keywords to that many videos but it would be kind of a pain to lose the ones that I have.
Here's an approach that should preserve existing keywords, captions, and titles:
1. Export the keywords (and perhaps captions and titles) of all your videos to a text file using the LR/Transporter plugin.
2. Remove all the videos from the catalog.
3. Import all the videos, picking up metadata from the .thm sidecars.
4. Use LR/Transporter to import the keywords from the text file of step 1.
The most recent version of the ListView plugin also provides a similar capability, optimized for expert Excel users.
Of course, make a backup of your catalog before you start.