OK, it's not a limitation of the eCS version, however, it is a requirement, just as in the thread you quoted. There is no requirement to use OpenOffice, however, while there is one for XLSX format.
Zoho Office does so, and runs nicely in Firefox 3.5.7 under eCS.
This isn't meant as a criticism of OOo for eCS. For people with a round-trip requirement for XLSX files, Zoho office is a reasonable application. I also think that OOo will eventually support XLSX format.