I don't have a clue why Excel 2013 created XML that 2007 could not read. I would not expect that to happen and I would be equally annoyed if it had happened to me.
If you still have a copy of the workbook in the state which failed to load in 2007, I would suggest sending it to Microsoft as a support incident. I am fairly certain that they would treat the generation of content by 2013 that was unreadable by 2007 as
a bug and would want to investigate and fix it.
I would, however, like to make 2 observations based on many years of experience:
a) If you are creating an Excel workbook that is to be used in different versions of Excel you should always do the development in the earliest version with which it will be used, to avoid potential backwards compatibility problems
b) For a major piece of work like this you should always have a good backup strategy so that if the workbook suffers a significant failure you can retrieve a previous state easily.
Hopefully you do have a backup even if it is a bit old.
You should be able, in 2007, to copy the cells from the worksheets in the backup file and paste special > comments into the version that has currently lost its comments.