When you attempt to open a Pages v5 or later document in Pages '08 or Pages '09, you will get the dialog about the missing index.xml file. That is because documents created in the newer versions of Pages do not have the same internal organization, and no longer use the XML-based index.xml file that was used in Pages '09 and earlier.
You can also get this missing index.xml dialog when you mistakenly close a Pages '08 document using the red traffic light on the document header. This does not quit Pages '08 (or '09) but keeps it running. If you then double click a Pages document in the new document format, the still older Pages will ambush that open and discover the document is not compatible. To prevent this dialog, you must always formally quit Pages '08/'09 on a Mac where you have mixed old and new generation documents — which sadly, without the use of different tag colors, have indistinguishable document icons. You will need instead to right-click on the document, and choose Open With : Pages (default) 8.1, or Pages (3.03), or Pages (4.3) as appropriate.
Pages has had the ability to open Pages '08 documents since Pages v5.6.2 on El Capitan. On new Macs with (currently) Pages v8.1, this remains true, and even the ability to open Pages '05 files. Although there is an export to Pages 09 document format, there is none for Pages '08, and one would have to export either in Rich Text or Word document format for backward compatibility with Pages '08.