You are correct as to what the manual says. However, higher capacity Micro SDHC cards will work.

The manual says what it says because at the time it was written, 8 GB Micro SDHC cards were the highest capacity available. Nokia, unable to officially test and certify non-existent larger cards, erred on the side of caution.

It would have been nice if Nokia had included some statement that higher capacity, standards compliant cards should work, but have not been tested, blah, blah, blah, no guarantees. The very idea probably sets the legal department to twitching though, so instead there are now many, many N8x0 owners who falsely believe they're living under an 8 GB ceiling.