What is the maximum voltage of a Lithium Ion Battery
The datasheet I'm reading says those batteries (like almost all other Li-Ion battery packs) have a charging voltage of 4.2V +/- 0.05V per cell. So two cell pack would be 8.4V plus or minus 0.1V.

So yes, it is within the tolerance range as specified by the manufacturer. But bear in mind you are supposed to aim for 8.4V! So if you aim for 8.5V and you are the smallest bit off then you will be out of tolerance.

My guess is that the manufacturer of your charger is simply saying the same thing as the manufacturer of your batteries: 8.5V is the maximum value in the range that the charger is specified to output.

Simple answer is that this seems fine to me.