What do you mean “open source”? XLS is the (previously proprietary) format used by older versions that’s (basically) just serialized structs. XLSX (which I see here) is the ZIP/XML based version based on an open standard[0][1]. LibreOffice opens those just fine from my experience; It’s the older binary format that LibreOffice can screw up with formatting.
Perhaps things have gotten better, but OOXML is not an open format by the normal usage of the word (text based does not mean open). OOXML was rushed through the standardization process, and was entirely created by one vendor (Microsoft).
Microsoft doesn't even bother to support the OOXML format completely, as the Wikipedia article notes.
You are right although as I understood it, the MS standard is exceedingly complicated, some would say in an attempt to "obfuscate" it.
In any case, this specific excel template states that it requires Excel 2007 and might misbehave if using any other program such as OpenOffice or Google Sheets:
> Please note that Microsoft Excel 2007 or newer is required. Other software like Open Office, Google Docs, and Numbers may import the Excel spreadsheet, but due to differences in the way they handle dates you should double-check that the calendars are correct.
Microsoft doesn't even bother to support the OOXML format completely, as the Wikipedia article notes.