I think you are right, there is a terminology conflict. Interestingly, you can work around the problem using where its in place of whose:
property abc : "Hello World"
property def : 1234
tell application id "com.latenightsw.ScriptDebugger7"
tell document 1
get script properties where its type = Unicode text
end tell
end tell
P.S. This issue will be addressed in the Script Debugger 7.0.9 maintenance release.