Do you have these instructions? They say 8mm too. But still Shimano used both sizes for the bolts at various times. Maybe yours isn't the original bolts or it's a version that wasn't out at the time of this document's publishing.

https://si.shimano.com/en/pdfs/si/F7...000-00-ENG.pdf

more documents that might be helpful are here.... https://si.shimano.com/en/manual/sea..._model=fc-m510