I figured out a workaround:
1. Download your data with Google Takeout
2. Open "starred.json" in a text editor that supports regex find/replace
3. Search for "^.*content" : ".*$"
4. Replace " "content" : """
[don't include the outside set of quotes; i.e. the end of the replace should have a double set of quotes, not triple. Include the spaces]
Test it out and play with it if you don't get quite the results you want, but a find/replace all enabled me to upload all my starred items without anything going into the description field in Pinboard.