WinRT: Native dialog with PickMultipleFilesAsync not supported on Windows Phone 8.1

I get errors from qwinrtfiledialoghelper.cpp such as:
Failed to open multi file picker (The request is not supported.)

when running code such as:
QFileDialog::getOpenFileNames(this);

Same sort of errors when it calls PickMultipleFilesAsync, PickSingleFileAsync, PickSaveFileAsync, and presumably all the others.

Doing a little looking around online, it appears that for Windows Phone functions such as PickSaveFileAndContinue are preferred over PickSaveFileAsync, which seems to be confirmed on: https://msdn.microsoft.com/en-us/library/windows/apps/xaml/dn614994.aspx?f=255&MSPPError=-2147217396 Though they don't outright say the async versions aren't supported, that's what it appears.