I think you are on to something, and I can make it work now for everyone. The odd thing is, my original way works fine for me.
This new file linked → ShowMeYourSecretsFixed.zip (2.2 KB)
has the DataSourceReference blank. When you upload it, it will fill in the proper value.
(or you can just figure out yours if you want)
<DataSources>
<DataSource Name="IHaveSecrets">
<DataSourceReference></DataSourceReference>
<rd:SecurityType>Integrated</rd:SecurityType>
<rd:DataSourceID>bb3095bd-d2b3-47be-9b22-8aa573add66d</rd:DataSourceID>
</DataSource>
</DataSources>
Happy playing.
Edit: After you upload it, redownload it with the proper value to keep report builder from screwing it up.