I know from experience that not all members of the IPIData interface are implemented for point lists. What you could try is to cast your Data member to an IPIData2 interface and try to use the Summaries2 method. The signature is almost the same, but it has no out parameter. It returns the NamedValues collection.
PISDK didn't implement bulk summary calls, they're not implemented as suggested by the exception.
We have to wait for AFSDK 2.6 to have bulk summary calls, even AFSDK 2.5 RDA doesn't have those.
Even with AFSDK 2.6 you'll need to have a PI Server backend >= 3.4.390 to make use of the newer bulk RPCs that are exposed, otherwise the summaries are processed per PIPoint.