The big feature that OSVVM uses is protected types.
Protected types are not a VHDL-2008 feature. They are older. VHDL-2000 / VHDL-2002.
Hence, from a marketing perspective, they may still claim VHDL-2008 support – meaning that they support the new stuff even while they do not support the older VHDL-2002 features.
Be sure to file bug reports, complain in Xilinx forums, and to your Xilinx Sales and Applications people. It is particularly important to tell them that the tools that Altera/Intel use do support OSVVM.