NOTE:. Ondemand does load all dependencies listed for any ondemand extension.
Gnumeric ondemand does load libxcb.tcz. Only applications should be required in the onboot.lst
Dependencies are loaded as an extenion loads and therefore do not need to be added to the onboot.lst or ondemand
If one is experiencing issues with dependencies then use AppsAudit, Update .dep files, followed by Fetch Missing Dependencies.
In your particular case, you already had downloaded libcxb.tcz, therefore the proper way would be to use AppsAudit Update .dep files or if you perfer manuall edit gnumeric.tcz.dep and add the libxcb.tcz. If one wishes to ignore the features of AppsAudit then for dependencies one should only use the download only or download + load options of AppBrowser.
Dependencies do not belong in onboot.lst or ondemand.lst By placing dependencies in onboot.lst is only slowing the boot process as such dependency is going to be handled twice by the system. Once as a by-product of loading gnumeric and second by its listing in onboot.